Confirm the commercial relationship first
The merchant owns provider selection, application, approval, fees and settlement terms. Technical work begins from the provider's current approved documentation.
Treat every response as untrusted
Server-side verification, idempotent order handling and clear success, pending and failed states protect the checkout from ambiguous browser returns.
Test the full order lifecycle
Authorisation, failure, cancellation, notification, refund responsibilities and reconciliation need documented ownership before launch.