Starting from $800, quoted per project once I have seen your codebase. I add sign-in and account management plus Stripe billing to a web app you already have: Stripe Checkout, subscription plans, webhooks that keep your database in sync with payment status, and a customer billing portal. You get the code in your own repository, a short setup note, and a test pass in Stripe test mode before anything goes live. Not included: a redesign, new product features, legal or tax set-up, or ongoing support after handover (available separately). I have built this stack into my own products, including BusinessPilotOS (Supabase, Stripe, Vercel). Message me with your stack and what you sell.