A billing bug is not like other bugs. A broken button annoys a user; a broken checkout charges them. So when I owned QA on subscription flows, I treated every rupee, dollar and cent like it was my own money moving.
I ran 100+ REST endpoints through Postman — schema rules, auth headers, pagination, rate limits — and then went after the quiet part: webhooks. I replayed duplicate Stripe events to check idempotency, matched subscription state across Stripe, the backend database and the UI with SQL, and walked every upgrade, downgrade and cancellation path by hand.
The result was the most boring sentence in QA: zero billing sync issues shipped. Boring is the whole point. Nobody ever writes a support ticket that says "my invoice was exactly right," and that silence is the review I work for.