If you're building an AI app (image generation, transcription, an agent, anything that calls a model) you've probably realized a flat "$10/month" doesn't work. Every action costs you real money in GPU/API spend, so a single power user can torch your margins. The answer is usage credits: users buy a balance, each action spends some.
Credits sound trivial. They are not. I've shipped about 10 small AI/SaaS apps, and the credit layer is where I got burned every single time. It took three patterns to fix it for good. Here they are, with copy-pasteable code for Next.js + Supabase + Stripe. Get these right and your billing won't oversell, double-charge, or strand a user's money.
The three things everyone gets wrong
Overdrawing. Two requests arrive at once, both read "balance = 1," both spend. Now the balance is negative and you gave away work for free.
Double-granting. Stripe retries webhooks (it will), and if you grant credits on every delivery, a $9 purchase becomes $18 of credits.
