diff --git a/buckaroo/builders/payments/in3_builder.py b/buckaroo/builders/payments/in3_builder.py
index 71e5dab..3df0ca8 100644
--- a/buckaroo/builders/payments/in3_builder.py
+++ b/buckaroo/builders/payments/in3_builder.py
@@ -1,8 +1,9 @@
 from typing import Dict, Any
 from .payment_builder import PaymentBuilder
+from .capabilities.authorize_capture_capable import AuthorizeCaptureCapable
 
 
-class In3Builder(PaymentBuilder):
+class In3Builder(PaymentBuilder, AuthorizeCaptureCapable):
     """Builder for IN3 payments with bank transfer capabilities."""
 
     def get_service_name(self) -> str:
@@ -12,8 +13,8 @@ def get_service_name(self) -> str:
     def get_allowed_service_parameters(self, action: str = "Pay") -> Dict[str, Any]:
         """Get the allowed service parameters for IN3 payments based on action."""
 
-        if action.lower() in ["pay"]:
-            return {
+        if action.lower() in ["pay", "authorize"]:
+            params = {
                 "billingCustomer": {
                     "type": list,
                     "required": True,
@@ -27,4 +28,16 @@ def get_allowed_service_parameters(self, action: str = "Pay") -> Dict[str, Any]:
                 "article": {"type": list, "required": True, "description": "IN3 articles"},
             }
 
+            # Authorize is only used for the ABN AMRO "Zakelijk op rekening"
+            # (business-on-account) flow, which the gateway routes via the
+            # required Route parameter (e.g. "abn_b2b"). Pay does not use it.
+            if action.lower() == "authorize":
+                params["route"] = {
+                    "type": str,
+                    "required": True,
+                    "description": 'Financing route, e.g. "abn_b2b" for ABN AMRO business',
+                }
+
+            return params
+
         return {}
diff --git a/examples/in3.py b/examples/in3.py
new file mode 100644
index 0000000..afe2157
--- /dev/null
+++ b/examples/in3.py
@@ -0,0 +1,145 @@
+#!/usr/bin/env python3
+"""Demo of the In3 payment method.
+
+Shows the regular ``.pay()`` flow and the ABN-AMRO "Zakelijk op rekening"
+(business-on-account) ``.authorize()`` / ``.capture()`` flow. All demos are gated
+on ``BUCKAROO_STORE_KEY`` / ``BUCKAROO_SECRET_KEY`` env vars.
+"""
+
+import os
+import sys
+
+# Add parent directory to Python path so the demo can import the SDK in-place.
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+from buckaroo.app import Buckaroo
+
+
+def _have_credentials() -> bool:
+    if not os.getenv("BUCKAROO_STORE_KEY") or not os.getenv("BUCKAROO_SECRET_KEY"):
+        print("⚠️  Set BUCKAROO_STORE_KEY and BUCKAROO_SECRET_KEY to run this demo")
+        return False
+    return True
+
+
+def demo_pay() -> None:
+    """Regular In3 flow: pay in three installments."""
+    print("\n1. In3 pay")
+    print("-" * 40)
+    if not _have_credentials():
+        return
+
+    try:
+        app = Buckaroo.from_env()
+        response = app.payments.create_payment(
+            "in3",
+            {
+                "currency": "EUR",
+                "amount": 250.00,
+                "description": "in3 pay demo",
+                "invoice": "IN3-PAY-001",
+                "return_url": "https://www.buckaroo.nl",
+                "return_url_cancel": "https://www.buckaroo.nl/cancel",
+                "return_url_error": "https://www.buckaroo.nl/error",
+                "return_url_reject": "https://www.buckaroo.nl/reject",
+                "service_parameters": {
+                    "article": [
+                        {"Description": "Widget", "Quantity": "2", "GrossUnitPrice": "125.00"},
+                    ],
+                    "billingCustomer": [_customer()],
+                    "shippingCustomer": [_customer()],
+                },
+            },
+        ).pay()
+        print(f"   status.code={response.status.code.code}  redirect={response.get_redirect_url()}")
+    except Exception as e:
+        print(f"   ❌ {e}")
+
+
+def demo_authorize_capture() -> None:
+    """ABN-AMRO "Zakelijk op rekening": authorize first, capture later.
+
+    This business-on-account flow uses ``.authorize()`` instead of ``.pay()`` and
+    requires the ``route`` service parameter set to ``"AbnB2b"``. Note the exact
+    Buckaroo field names: ``GrossUnitPrice`` (not Price), ``StreetNumber`` (not
+    house number), ``CountryCode`` (not country). Capture needs no service params.
+    """
+    print("\n2. In3 authorize / capture (ABN-AMRO Zakelijk op rekening)")
+    print("-" * 40)
+    if not _have_credentials():
+        return
+
+    try:
+        app = Buckaroo.from_env()
+
+        billing = _customer()
+        billing.update({"Category": "B2B", "CompanyName": "Acme B.V.", "CocNumber": "12345678"})
+
+        authorize_response = app.payments.create_payment(
+            "in3",
+            {
+                "currency": "EUR",
+                "amount": 250.00,
+                "description": "in3 authorize demo",
+                "invoice": "IN3-AUTH-001",
+                "return_url": "https://www.buckaroo.nl",
+                "return_url_cancel": "https://www.buckaroo.nl/cancel",
+                "return_url_error": "https://www.buckaroo.nl/error",
+                "return_url_reject": "https://www.buckaroo.nl/reject",
+                "service_parameters": {
+                    "route": "AbnB2b",
+                    "article": [
+                        {"Description": "Widget", "Quantity": "2", "GrossUnitPrice": "125.00"},
+                    ],
+                    "billingCustomer": [billing],
+                    "shippingCustomer": [_customer()],
+                },
+            },
+        ).authorize()
+        print(
+            f"   authorize: status.code={authorize_response.status.code.code}  "
+            f"key={authorize_response.key}"
+        )
+
+        # Once the order is confirmed, capture the authorized amount using the
+        # transaction key from the authorize response.
+        capture_response = app.payments.create_payment(
+            "in3",
+            {
+                "currency": "EUR",
+                "amount": 250.00,
+                "description": "in3 capture demo",
+                "invoice": "IN3-AUTH-001",
+            },
+        ).capture(original_transaction_key=authorize_response.key, amount=250.00)
+        print(f"   capture:   status.code={capture_response.status.code.code}")
+    except Exception as e:
+        print(f"   ❌ {e}")
+
+
+def _customer() -> dict:
+    """A minimal In3 customer using the exact Buckaroo field names."""
+    return {
+        "FirstName": "John",
+        "LastName": "Doe",
+        "Phone": "0612345678",
+        "Email": "john@example.com",
+        "Street": "Hoofdstraat",
+        "StreetNumber": "1",
+        "PostalCode": "1000AA",
+        "City": "Amsterdam",
+        "CountryCode": "NL",
+    }
+
+
+def main() -> None:
+    print("BUCKAROO SDK — IN3 DEMOS")
+    print("=" * 60)
+    demo_pay()
+    demo_authorize_capture()
+    print("\n" + "=" * 60)
+    print("done.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/feature/payments/test_in3.py b/tests/feature/payments/test_in3.py
index 96bfd7f..2071711 100644
--- a/tests/feature/payments/test_in3.py
+++ b/tests/feature/payments/test_in3.py
@@ -1,6 +1,8 @@
 """Feature test: in3 pay() round-trip through full stack with MockBuckaroo."""
 
 from tests.support.helpers import Helpers
+from tests.support.mock_request import BuckarooMockRequest
+from tests.support.recording_mock import recorded_service_parameters
 
 
 class TestIn3Feature:
@@ -13,7 +15,7 @@ def test_in3_pay_returns_pending_with_redirect(self, buckaroo, mock_strategy):
             payload_overrides={"amount": 25.00, "description": "Test in3"},
             service_params={
                 "article": [
-                    {"description": "Widget", "quantity": "2", "price": "12.50"},
+                    {"description": "Widget", "quantity": "2", "GrossUnitPrice": "12.50"},
                 ],
                 "billingCustomer": [
                     {"firstName": "John", "lastName": "Doe"},
@@ -23,3 +25,108 @@ def test_in3_pay_returns_pending_with_redirect(self, buckaroo, mock_strategy):
                 ],
             },
         )
+
+    def test_in3_authorize_returns_pending_with_redirect(self, buckaroo, mock_strategy):
+        def add_service_params(builder):
+            builder.add_parameter("route", "AbnB2b")
+            builder.add_parameter(
+                "article", [{"description": "Widget", "quantity": "2", "GrossUnitPrice": "12.50"}]
+            )
+            builder.add_parameter("billingCustomer", [{"firstName": "John", "lastName": "Doe"}])
+            builder.add_parameter("shippingCustomer", [{"firstName": "John", "lastName": "Doe"}])
+
+        Helpers.assert_action_returns_pending_with_redirect(
+            buckaroo,
+            mock_strategy,
+            method="in3",
+            invoice="INV-IN3-002",
+            action_name="Authorize",
+            call_method="authorize",
+            payload_overrides={"amount": 25.00, "description": "Test in3 authorize"},
+            extra_builder_setup=add_service_params,
+        )
+
+    def test_in3_authorize_then_capture_round_trip(self, buckaroo, mock_strategy):
+        auth_response_body = Helpers.pending_redirect_response("in3", "Authorize")
+        mock_strategy.queue(
+            BuckarooMockRequest.json("POST", "*/json/transaction", auth_response_body)
+        )
+
+        builder = buckaroo.payments.create_payment(
+            "in3",
+            Helpers.standard_payload(
+                invoice="INV-IN3-003", amount=25.00, description="Test in3 authorize"
+            ),
+        )
+        builder.add_parameter("route", "AbnB2b")
+        builder.add_parameter(
+            "article", [{"description": "Widget", "quantity": "2", "GrossUnitPrice": "12.50"}]
+        )
+        builder.add_parameter("billingCustomer", [{"firstName": "John", "lastName": "Doe"}])
+        builder.add_parameter("shippingCustomer", [{"firstName": "John", "lastName": "Doe"}])
+        authorize_response = builder.authorize()
+        assert authorize_response.is_pending()
+
+        capture_response_body = Helpers.success_response(
+            {
+                "Services": [{"Name": "in3", "Action": "Capture", "Parameters": []}],
+                "ServiceCode": "in3",
+            }
+        )
+        mock_strategy.queue(
+            BuckarooMockRequest.json("POST", "*/json/transaction", capture_response_body)
+        )
+        capture_response = builder.capture(
+            original_transaction_key=authorize_response.key, amount=25.00
+        )
+
+        assert capture_response.status.code.code == 190
+        assert capture_response.key == capture_response_body["Key"]
+
+    def test_in3_authorize_with_business_customer_returns_pending_with_redirect(
+        self, buckaroo, mock_strategy
+    ):
+        """ABN-AMRO "Zakelijk op rekening" (business-on-account) runs through the
+        In3 Authorize action with route="AbnB2b". The business customer is marked
+        by Category=B2B/CompanyName/CocNumber inside billingCustomer; those
+        sub-fields flow through the existing billingCustomer list group.
+        """
+
+        def add_service_params(builder):
+            builder.add_parameter("route", "AbnB2b")
+            builder.add_parameter(
+                "article", [{"description": "Widget", "quantity": "2", "GrossUnitPrice": "12.50"}]
+            )
+            builder.add_parameter(
+                "billingCustomer",
+                [
+                    {
+                        "Category": "B2B",
+                        "CompanyName": "Acme B.V.",
+                        "CocNumber": "12345678",
+                        "firstName": "John",
+                        "lastName": "Doe",
+                    }
+                ],
+            )
+            builder.add_parameter("shippingCustomer", [{"firstName": "John", "lastName": "Doe"}])
+
+        Helpers.assert_action_returns_pending_with_redirect(
+            buckaroo,
+            mock_strategy,
+            method="in3",
+            invoice="INV-IN3-004",
+            action_name="Authorize",
+            call_method="authorize",
+            payload_overrides={"amount": 25.00, "description": "Test in3 authorize B2B"},
+            extra_builder_setup=add_service_params,
+        )
+
+        billing_params = {
+            param["Name"]: param["Value"]
+            for param in recorded_service_parameters(mock_strategy)
+            if param["GroupType"] == "Billingcustomer"
+        }
+        assert billing_params["Category"] == "B2B"
+        assert billing_params["Companyname"] == "Acme B.V."
+        assert billing_params["Cocnumber"] == "12345678"
diff --git a/tests/unit/builders/payments/test_concrete_builders_contract.py b/tests/unit/builders/payments/test_concrete_builders_contract.py
index a66dbe8..181974a 100644
--- a/tests/unit/builders/payments/test_concrete_builders_contract.py
+++ b/tests/unit/builders/payments/test_concrete_builders_contract.py
@@ -144,6 +144,7 @@ def test_capability_method_present_and_callable(
 EXPECTED_CAPABILITIES: Dict[str, set] = {
     "creditcard": {EncryptedPayCapable, AuthorizeCaptureCapable},
     "riverty": {AuthorizeCaptureCapable},
+    "in3": {AuthorizeCaptureCapable},
     "ideal": {BankTransferCapabilities, InstantRefundCapable, FastCheckoutCapable},
     "paybybank": {BankTransferCapabilities, InstantRefundCapable, FastCheckoutCapable},
     "payconiq": {BankTransferCapabilities, InstantRefundCapable, FastCheckoutCapable},
diff --git a/tests/unit/builders/payments/test_in3_builder.py b/tests/unit/builders/payments/test_in3_builder.py
index fef7206..181ea7a 100644
--- a/tests/unit/builders/payments/test_in3_builder.py
+++ b/tests/unit/builders/payments/test_in3_builder.py
@@ -54,7 +54,24 @@ def test_get_allowed_service_parameters_pay_is_case_insensitive(client):
     )
 
 
-@pytest.mark.parametrize("action", ["Refund", "Capture", "Authorize", "UnknownAction"])
+def test_get_allowed_service_parameters_authorize_snapshot(client):
+    builder = In3Builder(client)
+
+    authorize = builder.get_allowed_service_parameters("Authorize")
+
+    # Authorize allows every Pay param plus the required Route parameter
+    # (In3 Authorize only exists for the ABN AMRO "Zakelijk op rekening" flow).
+    assert authorize == {
+        **builder.get_allowed_service_parameters("Pay"),
+        "route": {
+            "type": str,
+            "required": True,
+            "description": 'Financing route, e.g. "abn_b2b" for ABN AMRO business',
+        },
+    }
+
+
+@pytest.mark.parametrize("action", ["Refund", "Capture", "UnknownAction"])
 def test_get_allowed_service_parameters_non_pay_returns_empty(client, action):
     assert In3Builder(client).get_allowed_service_parameters(action) == {}