Payment flow bugs are the most expensive bugs in SaaS. A broken login is frustrating. A broken payment flow is lost revenue.
And payment flows are among the hardest to test comprehensively. They involve third-party services (Stripe, Paddle, LemonSqueezy), async webhooks, multiple currency and tax scenarios, subscription state machines, and edge cases that only appear with specific card types or regions.
AI coding tools make payment flow bugs more likely because they generate plausible integrations that miss critical edge cases. The Stripe checkout looks right. The webhook handler exists. But the handler doesn't verify the webhook signature. Or it doesn't handle duplicate events. Or it creates a subscription but doesn't record it in the local database.
The Payment Testing Checklist
Checkout Flow
Webhook Handling
Subscription State
Edge Cases
Automating Payment Flow Tests
Manual testing of payment flows requires test cards, test webhook endpoints, and careful state management. TestSprite automates this by testing the full payment flow against your staging environment, including Stripe test mode integration.
The agent tests checkout completion, error handling, subscription state transitions, and webhook processing as part of the comprehensive test suite. Payment flow bugs are caught on the PR, not discovered when a customer's card gets charged but their account doesn't upgrade.