Full Toss Payments integration — then one line broke all Korean checkouts

A developer integrated Toss Payments for Korean users alongside Paddle for international users in the FateSaju fortune-telling app, including widget rendering, server-side HMAC verification, webhook handling, and multilingual receipt emails across 8 locales. After shipping the full payment flow and cleaning up the design system, a single misconfigured config value was discovered that had been silently routing all Korean users to an unconfigured Paddle endpoint, breaking Korean checkouts entirely.