Payment Gateway Integration That Reconciles With Your Books

What does payment gateway integration involve?

A gateway integration connects your website, app or back-office software to a payment processor such as Stripe, Adyen, Authorize.Net, Braintree or Square. It creates a payment session, collects the card details through the processor's hosted fields or checkout page, receives the result, and records it in your own system.

Volume justifies the care. The Federal Reserve counted 166.1 billion card payments in the U.S. in 2022, worth about $10.4 trillion, so any error rate that looks small becomes a steady stream of support tickets.

Gateways differ in what they offer beyond the basic charge: saved payment methods, subscription billing, payment links, marketplace payouts, local methods such as ACH debit and bank redirects, and tax tools. Choosing on features you will actually use avoids paying for complexity.

How do you keep card data out of your systems?

Use the processor's hosted checkout or embedded fields so card numbers go straight to the processor. Your server then handles a token and not the card number. This reduces your PCI DSS burden, though it does not remove it: merchants using the lightest self-assessment, SAQ A, still carry security responsibilities such as protecting the page that embeds the payment form.

Never log full card numbers or CVV values, and keep secret API keys in environment configuration rather than in source.

Currency and tax add another layer. Amounts should be stored as integers in the smallest currency unit, as Stripe's API expects, so that rounding errors never creep into your totals. Keep the gateway's charge ID on every order, since support, refunds and reconciliation all start from that reference.

Why are webhooks the most important part?

The browser redirect after payment is not proof of payment. The reliable signal is the webhook your processor sends, for example payment_intent.succeeded on Stripe. Stripe documents that events can arrive more than once and in a different order from the one they were generated in, and it recommends tracking event IDs to detect duplicates.

Your handler should verify the Stripe-Signature header against the raw request body, return a 2xx quickly, and do the slower work, such as marking an invoice paid in accounting, from a queue. Stripe retries undelivered events for up to three days in live mode, so a short outage on your side does not have to lose a payment.

If you take payments in more than one place, such as an online store, an invoice portal and a phone order screen, route them all through the same payment service in your code. One implementation of refunds, receipts and webhooks is easier to secure and audit than three.

What else should the integration handle?

The checkout flow is a fraction of the total work. These are the cases that cause month-end problems:

• Refunds and partial refunds, matched back to the original invoice
• Disputes and chargebacks, with evidence deadlines tracked
• Failed recurring payments and retry rules for subscriptions
• Payout reconciliation: processor payouts bundle many charges and fees into one bank deposit
• Multi-currency pricing and rounding
• Sales tax or VAT calculation on the order

How do you test a payment integration safely?

Use the processor's test mode and test card numbers to trigger declines, 3D Secure challenges and disputes. Then replay duplicate and out-of-order webhooks against your handler. A launch checklist should include a small live transaction followed by a refund before real customers arrive.

Also test what happens when your accounting sync is down while payments continue. The payment should still be recorded, and the accounting entry should be queued and retried, never lost.