Money mistakes are unforgiving. A rounding error in a payment service isn't a minor bug — it's someone's salary, someone's rent. You don't get to "move fast and break things" when real money is on the line.
Our payment service embraced idempotency keys so duplicate requests couldn't double-charge anyone, circuit breakers so a downstream failure wouldn't cascade, and comprehensive audit logs so every cent could be traced.
Today it hums along quietly handling multi-currency payouts around the globe. Success, in this case, is when no one notices the complexity you wrestled into submission. The best payment system is one you never think about.