Building a Pay-Per-Use SaaS: Lessons from a Caption Tool

The original frustration was simple: I was paying €10/month for a caption generator and using it three times. That is over €3 per video for a tool that does a single, stateless job — take a video file, produce a subtitle file, done. So I built a pay-per-use alternative. What I did not expect was how much the billing model would reshape every technical decision from the ground up. The Architecture Difference A subscription SaaS and a pay-per-use SaaS look similar on the surface — users, auth, a product feature — but the payment and job model diverges significantly. Subscription model: User pays monthly → gets a "member" flag → unlimited (or quota-limited) access Auth middleware checks membership status Usage tracking is optional (for analytics, not billing) Pay-per-use model: User has a credit balance (or card on file) Every job deducts a defined cost before processing starts If balance is insufficient, the job is rejected before any compute runs The credit deduction is atomic with the