A payment gateway moves rupees once. It authorizes a card or a UPI request, settles the money, and fires a success webhook. That is where most integration guides stop, and that is exactly where the real work of a subscription business begins. Recurring revenue is not one payment repeated. It is a relationship you have to manage on a schedule, forever, and no gateway sells you that layer.
In India the gap is wider than the Stripe-world tutorials admit. You cannot just save a card and re-charge it whenever you like. You register a mandate the customer's bank approves, you live inside RBI's e-mandate rules, you respect the UPI Autopay per-debit cap, and you stamp a GST-correct tax invoice on every renewal. Get any one of those wrong and the debit fails or the invoice does not match the money in your account.
This guide is the full build, in the order we actually build it: what a billing system is versus a gateway, the India mandate problem, the data model, the subscription state machine, proration maths, dunning, GST-compliant recurring invoices, the self-serve portal, revenue analytics, and an honest build-vs-buy call. We write it from production, not a vendor deck. Where a number depends on the current rulebook or your own pricing, we say so in the text, and you should verify it against your own setup before you ship.
