diff --git a/buckaroo/builders/base_builder.py b/buckaroo/builders/base_builder.py
index 314b56b..54538cb 100644
--- a/buckaroo/builders/base_builder.py
+++ b/buckaroo/builders/base_builder.py
@@ -202,8 +202,10 @@ def add_parameter(
         # Convert value to string for API compatibility
         str_value = str(value).lower() if isinstance(value, bool) else str(value)
 
+        # Top-level names keep internal caps ("CustomerFirstName"); grouped
+        # names stay flattened via capitalize().
         parameter = Parameter(
-            name=key.capitalize(),
+            name=key.capitalize() if group_type else _upper_first(key),
             value=str_value,
             group_type=_upper_first(group_type) if group_type else None,
             group_id=group_id,
diff --git a/buckaroo/builders/solutions/credit_management_builder.py b/buckaroo/builders/solutions/credit_management_builder.py
index 62619fa..3189041 100644
--- a/buckaroo/builders/solutions/credit_management_builder.py
+++ b/buckaroo/builders/solutions/credit_management_builder.py
@@ -111,9 +111,9 @@ class CreditManagementBuilder(SolutionBuilder):
     # type, then caller-friendly field name -> wire field name (matched
     # case-insensitively against the already-added ``Parameter``s).
     # DebtorInfo maps the debtor's code to wire name "Debtorcode" (not
-    # "Code", unlike AddOrUpdateDebtor/CreateInvoice); the wire name is run
-    # through ``.capitalize()`` like every other parameter name, so the
-    # camelCase written here does not survive as-is.
+    # "Code", unlike AddOrUpdateDebtor/CreateInvoice); as a grouped parameter
+    # its wire name is run through ``.capitalize()``, so the camelCase written
+    # here does not survive as-is.
     _WIRE_NAMES: Dict[str, Dict[str, Dict[str, str]]] = {
         "debtorinfo": {"Debtor": {"code": "DebtorCode"}},
     }
diff --git a/tests/feature/payments/test_banking.py b/tests/feature/payments/test_banking.py
index b3b6ee8..69efceb 100644
--- a/tests/feature/payments/test_banking.py
+++ b/tests/feature/payments/test_banking.py
@@ -51,9 +51,9 @@ def test_banking_payment_order_returns_success(self, buckaroo, mock_strategy):
         service = sent["Services"]["ServiceList"][0]
         assert service["Name"] == "Banking"
         assert service["Action"] == "PaymentOrder"
-        assert {"Name": "Accountholdername", "Value": "Arensman"} in [
+        assert {"Name": "AccountHolderName", "Value": "Arensman"} in [
             {"Name": p["Name"], "Value": p["Value"]} for p in service["Parameters"]
         ]
-        assert {"Name": "Iban", "Value": "NL44RABO0123456789"} in [
+        assert {"Name": "IBAN", "Value": "NL44RABO0123456789"} in [
             {"Name": p["Name"], "Value": p["Value"]} for p in service["Parameters"]
         ]
diff --git a/tests/feature/payments/test_giftcards.py b/tests/feature/payments/test_giftcards.py
index ea77dab..5cb5c35 100644
--- a/tests/feature/payments/test_giftcards.py
+++ b/tests/feature/payments/test_giftcards.py
@@ -21,9 +21,10 @@ def test_giftcards_pay_sends_cardnumber_and_pin_on_the_wire(
     ):
         """service_parameters dict must reach ServiceList[0].Parameters.
 
-        The SDK capitalizes parameter names, so ``"PIN"`` becomes ``"Pin"``
-        on the wire. Assert both the value pairing and the presence of both
-        keys so a builder that silently drops service_parameters would fail.
+        Top-level param names keep their internal casing, so ``"PIN"`` reaches
+        the wire as ``"PIN"``. Assert both the value pairing and the presence
+        of both keys so a builder that silently drops service_parameters would
+        fail.
         """
         recording_mock.queue(
             BuckarooMockRequest.json(
@@ -44,7 +45,7 @@ def test_giftcards_pay_sends_cardnumber_and_pin_on_the_wire(
         assert recorded_action(recording_mock) == "Pay"
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(recording_mock)}
         assert params.get("Cardnumber") == "1234567890123456"
-        assert params.get("Pin") == "1234"
+        assert params.get("PIN") == "1234"
 
     def test_giftcards_pay_redirect_mode_omits_card_parameters(
         self, recording_buckaroo, recording_mock
diff --git a/tests/feature/payments/test_klarna.py b/tests/feature/payments/test_klarna.py
index faac666..81ff1f0 100644
--- a/tests/feature/payments/test_klarna.py
+++ b/tests/feature/payments/test_klarna.py
@@ -58,7 +58,7 @@ def test_klarna_pay_as_capture_attaches_data_request_key(self, buckaroo, mock_st
         body = json.loads(mock_strategy.calls[-1]["data"])
         services = body["Services"]["ServiceList"]
         names = [p["Name"] for p in services[0]["Parameters"]]
-        assert "Datarequestkey" in names
+        assert "DataRequestKey" in names
 
     def test_klarna_cancel_reservation_posts_data_request_key(self, buckaroo, mock_strategy):
         """Follow-up actions carry the Buckaroo DataRequestKey (no reservation
@@ -79,7 +79,7 @@ def test_klarna_cancel_reservation_posts_data_request_key(self, buckaroo, mock_s
         assert service["Action"] == "CancelReservation"
         assert "OriginalTransactionKey" not in body
         names = [p["Name"] for p in service["Parameters"]]
-        assert "Datarequestkey" in names
+        assert "DataRequestKey" in names
 
     def test_klarna_update_reservation_dispatches_to_data_request(self, buckaroo, mock_strategy):
         response_body = Helpers.pending_redirect_response("klarna", action="UpdateReservation")
@@ -136,5 +136,5 @@ def test_klarna_pay_carries_shipping_details_on_transaction(self, buckaroo, mock
         service = json.loads(mock_strategy.calls[-1]["data"])["Services"]["ServiceList"][0]
         assert service["Action"] == "Pay"
         names = [p["Name"] for p in service["Parameters"]]
-        assert "Datarequestkey" in names
-        assert "Trackingnumber" in names
+        assert "DataRequestKey" in names
+        assert "TrackingNumber" in names
diff --git a/tests/feature/solutions/test_credit_management.py b/tests/feature/solutions/test_credit_management.py
index 10dfe45..9f957ea 100644
--- a/tests/feature/solutions/test_credit_management.py
+++ b/tests/feature/solutions/test_credit_management.py
@@ -67,9 +67,9 @@ def test_create_invoice_posts_service_parameters_and_returns_invoice_key(
         }
         assert ("Invoice", "") not in params
         assert ("Currency", "") not in params
-        assert params[("Invoiceamount", "")] == "250.00"
-        assert params[("Duedate", "")] == "2026-09-01"
-        assert params[("Schemekey", "")] == "SCHEME-2"
+        assert params[("InvoiceAmount", "")] == "250.00"
+        assert params[("DueDate", "")] == "2026-09-01"
+        assert params[("SchemeKey", "")] == "SCHEME-2"
         assert params[("Code", "Debtor")] == "DEBTOR-999"
 
     def test_create_invoice_without_required_params_raises_before_wire(self, buckaroo):
@@ -201,7 +201,7 @@ def test_debtor_file_actions_post_debtor_file_guid(
         assert response.status.code.code == 190
         assert recorded_action(mock_strategy) == action
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Debtorfileguid"] == "FILE-GUID-999"
+        assert params["DebtorFileGuid"] == "FILE-GUID-999"
 
     @pytest.mark.parametrize("method_name", ["resume_debtor_file", "pause_debtor_file"])
     def test_debtor_file_actions_without_debtor_file_guid_raise_before_wire(
@@ -271,7 +271,7 @@ def test_create_credit_note_posts_original_invoice_and_debtor(self, buckaroo, mo
             for p in recorded_service_parameters(mock_strategy)
         }
         assert ("Invoice", "") not in params
-        assert params[("Originalinvoicenumber", "")] == "INV-999"
+        assert params[("OriginalInvoiceNumber", "")] == "INV-999"
         assert params[("Code", "Debtor")] == "DEBTOR-999"
 
     def test_create_credit_note_without_debtor_raises_before_wire(self, buckaroo):
@@ -314,7 +314,7 @@ def test_add_or_update_product_lines_posts_indexed_articles(self, buckaroo, mock
             (p["Name"], p["GroupType"], p["GroupID"]): p["Value"]
             for p in recorded_service_parameters(mock_strategy)
         }
-        assert params[("Invoicekey", "", "")] == "INVK-999"
+        assert params[("InvoiceKey", "", "")] == "INVK-999"
         assert params[("Productid", "ProductLine", "1")] == "SKU-1"
         assert params[("Productname", "ProductLine", "1")] == "Widget"
         assert params[("Quantity", "ProductLine", "1")] == "2"
@@ -365,13 +365,13 @@ def test_create_payment_plan_posts_installment_and_dossier_fields(
         assert request["Description"] == "3-month plan"
 
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Includedinvoicekey"] == "INVK-999"
-        assert params["Dossiernumber"] == "DOSSIER-999"
-        assert params["Startdate"] == "2026-09-01"
+        assert params["IncludedInvoiceKey"] == "INVK-999"
+        assert params["DossierNumber"] == "DOSSIER-999"
+        assert params["StartDate"] == "2026-09-01"
         assert params["Interval"] == "Month"
-        assert params["Paymentplancostamount"] == "5.00"
-        assert params["Recipientemail"] == "debtor@example.com"
-        assert params["Installmentcount"] == "3"
+        assert params["PaymentPlanCostAmount"] == "5.00"
+        assert params["RecipientEmail"] == "debtor@example.com"
+        assert params["InstallmentCount"] == "3"
         assert "Description" not in params
 
     def test_create_payment_plan_without_included_invoice_key_raises_before_wire(self, buckaroo):
@@ -408,7 +408,7 @@ def test_terminate_payment_plan_posts_included_invoice_key(self, buckaroo, mock_
         assert response.status.code.code == 190
         assert recorded_action(mock_strategy) == "TerminatePaymentPlan"
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Includedinvoicekey"] == "INVK-999"
+        assert params["IncludedInvoiceKey"] == "INVK-999"
 
     def test_terminate_payment_plan_without_included_invoice_key_raises_before_wire(self, buckaroo):
         builder = buckaroo.solutions.create_solution("creditmanagement")
@@ -473,7 +473,7 @@ def test_create_combined_invoice_combines_into_ideal_pay(self, buckaroo, mock_st
             (p["Name"], p["GroupType"]): p["Value"] for p in services[1]["Parameters"]
         }
         assert ("Invoice", "") not in invoice_params
-        assert invoice_params[("Invoiceamount", "")] == "95.00"
+        assert invoice_params[("InvoiceAmount", "")] == "95.00"
         assert invoice_params[("Code", "Debtor")] == "DEBTOR-999"
 
         assert response.get_service_parameter("InvoiceKey") == "INVK-COMBINED-1"
diff --git a/tests/feature/solutions/test_emandate.py b/tests/feature/solutions/test_emandate.py
index e43750b..df56a4c 100644
--- a/tests/feature/solutions/test_emandate.py
+++ b/tests/feature/solutions/test_emandate.py
@@ -76,9 +76,9 @@ def test_create_mandate_posts_service_parameters_and_returns_mandate_id(
         assert recorded_action(mock_strategy) == "CreateMandate"
 
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Debtorreference"] == "DEBTOR-999"
-        assert params["Sequencetype"] == "1"
-        assert params["Purchaseid"] == "PUR-1"
+        assert params["DebtorReference"] == "DEBTOR-999"
+        assert params["SequenceType"] == "1"
+        assert params["PurchaseId"] == "PUR-1"
 
     def test_create_mandate_without_debtor_reference_raises_before_wire(self, buckaroo):
         builder = buckaroo.solutions.create_solution("emandate")
@@ -114,7 +114,7 @@ def test_status_posts_mandate_id_and_returns_status(self, buckaroo, mock_strateg
         assert recorded_action(mock_strategy) == "GetStatus"
 
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Mandateid"] == "MND-777"
+        assert params["MandateId"] == "MND-777"
 
     def test_status_without_mandate_id_raises_before_wire(self, buckaroo):
         builder = buckaroo.solutions.create_solution("emandate")
@@ -157,8 +157,8 @@ def test_modify_mandate_posts_service_parameters_and_returns_mandate_id(
         assert recorded_action(mock_strategy) == "ModifyMandate"
 
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Mandateid"] == "MND-555"
-        assert params["Maxamount"] == "1000.00"
+        assert params["MandateId"] == "MND-555"
+        assert params["MaxAmount"] == "1000.00"
 
     def test_modify_mandate_without_mandate_id_raises_before_wire(self, buckaroo):
         builder = buckaroo.solutions.create_solution("emandate")
@@ -200,8 +200,8 @@ def test_cancel_mandate_posts_service_parameters_and_returns_response(
         assert recorded_action(mock_strategy) == "CancelMandate"
 
         params = {p["Name"]: p["Value"] for p in recorded_service_parameters(mock_strategy)}
-        assert params["Mandateid"] == "MND-321"
-        assert params["Purchaseid"] == "PUR-2"
+        assert params["MandateId"] == "MND-321"
+        assert params["PurchaseId"] == "PUR-2"
 
     def test_cancel_mandate_without_mandate_id_raises_before_wire(self, buckaroo):
         builder = buckaroo.solutions.create_solution("emandate")
diff --git a/tests/feature/solutions/test_marketplaces.py b/tests/feature/solutions/test_marketplaces.py
index b418bb0..b78ef04 100644
--- a/tests/feature/solutions/test_marketplaces.py
+++ b/tests/feature/solutions/test_marketplaces.py
@@ -85,7 +85,7 @@ def test_split_combines_marketplaces_into_ideal_pay(self, buckaroo, mock_strateg
         split = {
             (p["Name"], p["GroupType"], p["GroupID"]): p["Value"] for p in services[1]["Parameters"]
         }
-        assert split[("Daysuntiltransfer", "", "")] == "2"
+        assert split[("DaysUntilTransfer", "", "")] == "2"
         assert split[("Amount", "Marketplace", "")] == "10.00"
         assert split[("Accountid", "Seller", "1")] == "789C60F316D24B088ACD471"
         assert split[("Accountid", "Seller", "2")] == "369C60F316D24B088ACD238"
@@ -145,7 +145,7 @@ def test_transfer_with_splits_posts_grouped_params(self, buckaroo, mock_strategy
         assert params[("Amount", "Marketplace", "")] == "10.00"
         assert params[("Accountid", "Seller", "1")] == "789C60F316D24B088ACD471"
         # DaysUntilTransfer never applies to a Transfer.
-        assert not any(name == "Daysuntiltransfer" for name, _, _ in params)
+        assert not any(name == "DaysUntilTransfer" for name, _, _ in params)
 
     def test_transfer_requires_original_key(self, buckaroo):
         with pytest.raises(ValueError):
@@ -245,10 +245,10 @@ def test_manual_transfer_posts_accounts_and_amount(self, buckaroo, mock_strategy
         assert "AmountDebit" not in body
 
         params = {p["Name"]: p["Value"] for p in service["Parameters"]}
-        assert params["Fromaccountid"] == "AAAAAAAAAAAAAAAAAAA"
-        assert params["Toaccountid"] == "BBBBBBBBBBBBBBBBBBB"
-        assert params["Fromdescription"] == "Deduction monthly fee"
-        assert params["Todescription"] == "Monthly fee third party ABC"
+        assert params["FromAccountId"] == "AAAAAAAAAAAAAAAAAAA"
+        assert params["ToAccountId"] == "BBBBBBBBBBBBBBBBBBB"
+        assert params["FromDescription"] == "Deduction monthly fee"
+        assert params["ToDescription"] == "Monthly fee third party ABC"
         assert response.status.code.code == 190
 
     def test_manual_transfer_requires_all_fields(self, buckaroo):
diff --git a/tests/unit/builders/payments/capabilities/test_fast_checkout_capable.py b/tests/unit/builders/payments/capabilities/test_fast_checkout_capable.py
index b86f507..67dc911 100644
--- a/tests/unit/builders/payments/capabilities/test_fast_checkout_capable.py
+++ b/tests/unit/builders/payments/capabilities/test_fast_checkout_capable.py
@@ -102,4 +102,4 @@ def test_validate_false_skips_parameter_validation(self):
 
         service = recorded_request(mock)["Services"]["ServiceList"][0]
         parameter_names = [p["Name"] for p in (service.get("Parameters") or [])]
-        assert "Someunknownparam" in parameter_names
+        assert "SomeUnknownParam" in parameter_names
diff --git a/tests/unit/builders/payments/test_giftcards_builder.py b/tests/unit/builders/payments/test_giftcards_builder.py
index ea0c5af..1561a2f 100644
--- a/tests/unit/builders/payments/test_giftcards_builder.py
+++ b/tests/unit/builders/payments/test_giftcards_builder.py
@@ -187,7 +187,7 @@ def test_refund_build_validates_when_intersolve_email_absent(client):
 
     service = request.to_dict()["Services"]["ServiceList"][0]
     names = {p["Name"] for p in service["Parameters"]}
-    assert "Lastname" in names
+    assert "LastName" in names
     assert "Email" not in names
 
 
@@ -205,7 +205,7 @@ def test_refund_build_keeps_intersolve_email_when_supplied(client):
 
     service = request.to_dict()["Services"]["ServiceList"][0]
     names = {p["Name"] for p in service["Parameters"]}
-    assert {"Lastname", "Email"} <= names
+    assert {"LastName", "Email"} <= names
 
 
 def test_get_allowed_service_parameters_refund_is_case_insensitive(client):
diff --git a/tests/unit/builders/payments/test_klarna_builder.py b/tests/unit/builders/payments/test_klarna_builder.py
index 739e46c..b2bcf7d 100644
--- a/tests/unit/builders/payments/test_klarna_builder.py
+++ b/tests/unit/builders/payments/test_klarna_builder.py
@@ -276,7 +276,7 @@ def test_posts_cancel_reservation_to_data_request_without_original_key(self):
         sent = recorded_request(mock)
         assert "OriginalTransactionKey" not in sent
         names = [p["Name"] for p in sent["Services"]["ServiceList"][0]["Parameters"]]
-        assert "Datarequestkey" in names
+        assert "DataRequestKey" in names
         assert response.key == "KL-CAN-1"
         mock.assert_all_consumed()
 
diff --git a/tests/unit/builders/solutions/test_credit_management_builder.py b/tests/unit/builders/solutions/test_credit_management_builder.py
index a4d0574..0eb477b 100644
--- a/tests/unit/builders/solutions/test_credit_management_builder.py
+++ b/tests/unit/builders/solutions/test_credit_management_builder.py
@@ -338,7 +338,7 @@ def test_debtor_file_actions_post_debtor_file_guid_to_data_request(
 
     service = recorded_request(mock_strategy)["Services"]["ServiceList"][0]
     params = {p["Name"]: p["Value"] for p in service["Parameters"]}
-    assert params["Debtorfileguid"] == "FILE-GUID-001"
+    assert params["DebtorFileGuid"] == "FILE-GUID-001"
 
 
 @pytest.mark.parametrize("method_name", ["resume_debtor_file", "pause_debtor_file"])
@@ -439,9 +439,9 @@ def test_create_credit_note_posts_invoice_top_level_and_debtor_group_to_data_req
 
     service = request["Services"]["ServiceList"][0]
     params = {(p["Name"], p["GroupType"]): p["Value"] for p in service["Parameters"]}
-    assert params[("Originalinvoicenumber", "")] == "INV-001"
-    assert params[("Invoicedate", "")] == "2026-07-16"
-    assert params[("Invoiceamount", "")] == "10.0"
+    assert params[("OriginalInvoiceNumber", "")] == "INV-001"
+    assert params[("InvoiceDate", "")] == "2026-07-16"
+    assert params[("InvoiceAmount", "")] == "10.0"
     assert params[("Code", "Debtor")] == "DEBTOR-001"
 
 
@@ -500,7 +500,7 @@ def test_add_or_update_product_lines_posts_indexed_grouped_articles(client, mock
 
     service = recorded_request(mock_strategy)["Services"]["ServiceList"][0]
     params = {(p["Name"], p["GroupType"], p["GroupID"]): p["Value"] for p in service["Parameters"]}
-    assert params[("Invoicekey", "", "")] == "INVK-001"
+    assert params[("InvoiceKey", "", "")] == "INVK-001"
     assert params[("Productid", "ProductLine", "1")] == "SKU-1"
     assert params[("Productname", "ProductLine", "1")] == "Widget"
     assert params[("Quantity", "ProductLine", "1")] == "2"
@@ -669,13 +669,13 @@ def test_create_payment_plan_posts_to_data_request(client, mock_strategy):
 
     service = request["Services"]["ServiceList"][0]
     params = {p["Name"]: p["Value"] for p in service["Parameters"]}
-    assert params["Includedinvoicekey"] == "INVK-001"
-    assert params["Dossiernumber"] == "DOSSIER-1"
-    assert params["Startdate"] == "2026-09-01"
+    assert params["IncludedInvoiceKey"] == "INVK-001"
+    assert params["DossierNumber"] == "DOSSIER-1"
+    assert params["StartDate"] == "2026-09-01"
     assert params["Interval"] == "Month"
-    assert params["Paymentplancostamount"] == "5.00"
-    assert params["Recipientemail"] == "debtor@example.com"
-    assert params["Installmentcount"] == "3"
+    assert params["PaymentPlanCostAmount"] == "5.00"
+    assert params["RecipientEmail"] == "debtor@example.com"
+    assert params["InstallmentCount"] == "3"
     assert "Description" not in params
 
 
@@ -720,7 +720,7 @@ def test_terminate_payment_plan_posts_to_data_request(client, mock_strategy):
 
     service = recorded_request(mock_strategy)["Services"]["ServiceList"][0]
     params = {p["Name"]: p["Value"] for p in service["Parameters"]}
-    assert params["Includedinvoicekey"] == "INVK-001"
+    assert params["IncludedInvoiceKey"] == "INVK-001"
 
 
 def test_terminate_payment_plan_raises_when_included_invoice_key_missing(client):
diff --git a/tests/unit/builders/test_base_builder.py b/tests/unit/builders/test_base_builder.py
index 9e31d2f..93dda9c 100644
--- a/tests/unit/builders/test_base_builder.py
+++ b/tests/unit/builders/test_base_builder.py
@@ -298,6 +298,23 @@ def test_add_parameter_flat_capitalizes_name_and_stringifies_value():
     ]
 
 
+def test_add_parameter_flat_keeps_internal_capitals():
+    # Top-level names keep internal caps: "customerFirstName" -> "CustomerFirstName".
+    builder = populate_required_fields(_make_builder(), amount=10.50)
+    builder.add_parameter("customerFirstName", "Jan")
+
+    request = builder.build(validate=False).to_dict()
+    service = request["Services"]["ServiceList"][0]
+    assert service["Parameters"] == [
+        {
+            "Name": "CustomerFirstName",
+            "GroupType": "",
+            "GroupID": "",
+            "Value": "Jan",
+        }
+    ]
+
+
 def test_add_parameter_grouped_sets_group_type_and_group_id():
     builder = populate_required_fields(_make_builder(), amount=10.50)
     builder.add_parameter("firstName", "Jane", group_type="customer", group_id="7")