Why Payment Integration Requires Special Care
Payment integrations are among the most consequential code you will write. Errors can mean failed transactions, double charges, or security vulnerabilities. The financial and reputational cost of getting payment integration wrong far exceeds the cost of getting it right the first time. This guide covers the practices that distinguish robust payment integrations from fragile ones.
Authentication and Security
API keys must be treated like passwords. Store them in environment variables or secrets management services, never in source code or version control. Rotate keys regularly and immediately upon any suspected exposure. Use separate API keys for development, staging, and production environments. Implement IP allowlisting for server-to-server payment API calls wherever the API supports it.
Idempotency
Idempotency is the property of an operation that can be executed multiple times with the same result as a single execution. For payment operations, this means that if a network timeout causes you to retry a payment request, the retry will not result in a duplicate charge. Implement idempotency keys for all payment creation requests and store them persistently so retries send the same key.
Webhook Handling
Webhooks are the primary mechanism for payment status updates. Implement webhook endpoints that acknowledge receipt immediately (return 200 before processing), verify webhook signatures to prevent spoofing, process webhooks asynchronously to avoid timeout issues, and handle duplicate webhook delivery gracefully (webhooks may be delivered more than once).
Error Handling
Payment APIs use error codes that carry specific meaning. Card declined errors need different handling from network timeout errors. Implement specific error handling for common cases (insufficient funds, expired card, invalid CVV), generic error handling for unexpected cases, and logging that captures enough context to debug production issues without storing sensitive payment data.
Testing
Use your payment provider's sandbox environment to test all expected flows and edge cases before production deployment. Test specifically: successful payment, declined payment, timeout handling, webhook delivery and processing, refund flows, and subscription lifecycle events. Automated integration tests against the sandbox protect against regressions as you continue developing.