Payment processing web app

Next.js + Gatsby for a payments product that integrated third-party gateways. Most of the work was the unhappy paths.

gateways integrated

shipped first

unhappy paths

bounded

spinner lifetime

Failed webhooks, retried captures, mobile responsiveness on devices we couldn't physically test on, and load-time tuning for low-bandwidth users — those were the actual job. The happy path was 40 lines. The rest was everything that decides whether someone trusts you with their card next time.

I learned that in money flows, the loading spinner is a lie until you can prove the transaction never silently succeeded.

{ what I did }

Shipped a Next.js + Gatsby frontend on top of third-party gateways.
Built deterministic state machines for retry/cancel/refund flows.
Tuned low-bandwidth performance for non-flagship-device users.

{ lessons }

In money flows, the loading spinner is a lie until you can prove the transaction never silently succeeded.
Mobile responsiveness on a payment screen is a regulatory feature, not a polish step.

{ stack }

Next.js
Gatsby
API Integration
React
Payments