Integrating Payment Gateways: What Breaks and Why

Every payment gateway's documentation makes integration look like an afternoon of work: create an account, drop in an API key, call a function, done. That afternoon's work is the easy 80%. The other 20% — the part that doesn't show up in the quick-start guide — is where most of the real engineering time actually goes, and it's where we've seen more launches slip than anywhere else in an e-commerce build.

What the quick-start guide doesn't cover

• Webhook reliability — what happens when the payment succeeds but the confirmation webhook to your server fails or arrives twice, and how your system avoids double-fulfilling or silently missing the order.
• Handling partial failures gracefully — a payment that's pending, not yet confirmed, or requires an additional verification step the user has to complete.
• Currency and rounding edge cases, especially for international customers where a gateway's rounding rules don't quite match your own pricing logic.
• Refunds and disputes as first-class flows, not an afterthought bolted on once a customer actually asks for money back.
• Reconciliation — making sure what your system thinks was charged actually matches what the gateway and your bank statement say, at scale, without someone manually checking.

Why this connects directly to trust

A checkout flow is the single moment in your product where a customer is handing over money and trusting you with their card details. A confusing error message, a payment that appears to fail but actually succeeded, or a page that hangs after clicking "pay" does more damage to trust in ten seconds than most other parts of a product combined.

The gateway handles moving the money. Your job is handling every state in between — pending, failed, duplicate, delayed — cleanly enough that the customer never has to wonder what actually happened.

What we actually test before launch

Every payment integration we ship gets tested against failure states deliberately, not just the happy path — declined cards, timeouts, duplicate webhook deliveries, and what the customer sees at each point. This is the same discipline behind not skipping QA, applied to the one part of a product where a missed edge case has a direct dollar cost attached to it, not just an annoyed user.