Solid writeup. Two gaps worth closing on that setup. Razorpay retries webhooks on any non-2xx or timeout and can deliver the same event more than once, so the handler has to be idempotent, dedupe on the payment id or event id with a unique constraint, otherwise a retry double-upgrades the account. Also return 200 fast and do the Supabase write async, the webhook times out in a few seconds and a slow write under load just triggers more retries. Second, slowapi is in-memory, so the moment you run more than one Render instance or more than one uvicorn worker each process keeps its own counter and your real limit is Nx what you set, plus it resets on every deploy. For that to actually hold you need a shared store like Redis or rate limiting at the edge.