Integrate Secure Payment Gateways

Guides secure, reliable payment-processing integration across Stripe, PayPal, and Square: checkout flows and payment forms, subscription billing and recurring payments, webhook handling for payment events, PCI compliance, and payment error and retry handling. Its critical requirements center on webhook security and idempotency - always verify webhook signatures with official SDKs, never modify the raw request body before verification since JSON middleware breaks signature validation, store event IDs to make handlers idempotent because providers don't guarantee single delivery, return a 2xx response within 200ms before doing expensive work, and re-fetch payment status from the provider API rather than trusting the webhook payload alone. On PCI, it requires tokenization APIs (Stripe Elements, PayPal SDK) instead of ever touching raw card numbers, server-side payment verification via direct provider API calls, and hard separation between test and production credentials. It also insists on testing in sandbox/test mode first with an explicit, clear migration path to production, rather than developing directly against live payment credentials. Edge cases it explicitly calls out include disputes and refunds, not just failed payments on the checkout happy path - and the signature-verification requirement is grounded in the fact that Stripe's and PayPal's own SDKs already include HMAC-based signature verification, so hand-rolled verification is rarely necessary.

Use it when integrating or reviewing payment processing - checkout, subscriptions, webhooks, or PCI-sensitive flows. It grounds its rules in real-world failure patterns: a payment processor collapsing under a traffic spike causing webhook-queue backups and revenue loss, out-of-order webhooks breaking non-idempotent Lambda functions, price manipulation on unencrypted payment buttons, test cards accepted on live sites from misconfiguration, and a skipped webhook-signature check flooding a system with malicious requests.

Input: a payment feature to build or review - checkout, subscription billing, or webhook handling. Output: payment integration code with error handling, webhook endpoint implementations, a database schema for payment records, a PCI-compliance security checklist, test payment scenarios covering edge cases, and environment-variable configuration - always using official provider SDKs, with both server- and client-side code where needed.

Grounded in Stripe official docs, PayPal Security Guidelines, and the OWASP Testing Guide, plus production retrospectives of real payment-processing failures. Recommends official SDKs (Stripe, PayPal, Square) over hand-rolled integrations for both server-side and client-side code.

Developers implementing or auditing Stripe, PayPal, or Square payment integrations who need webhook security, idempotency, and PCI compliance handled correctly from the start.