I run Kill Bill in production as a real subscription engine, not a checkout wrapper. Catalog XML, plan phases, IN_ADVANCE / IN_ARREAR (postpaid), usage, add-ons, invoices, auto-renewal, and overdue / block-unblock are first-class — not a bolt-on. Multi-tenant works for a platform with many customers. I also love the plugin system. Payment, invoice, and entitlement hooks let me extend Kill Bill without forking the core. The one I use most: an invoice plugin that changes line items at generation time, so we can apply discounts when the invoice is built — not in a messy after-the-fact patch. On my team I was the person who actually learned how Kill Bill behaves (catalog versions, charged-through dates, requestedDate, blocking states) and then introduced most of those billing features — auto-renewal, add-ons, postpaid, block/unblock, runtime invoice discounts — because nobody else here knew the engine that well. I can read the source, prove a charge or a date, then post a full repro on killbilling-users. People there (and on GitHub) answer with code and issues, not marketing. I am still learning it. That is the point: it is deep enough that there is always more to understand. We are not paying Kill Bill a SaaS bill. The price is engineering time: I had to own catalog, requestedDate, overdue, and plugins before the invoices were trustworthy. Stripe Billing would have been more features on day one and a real invoice to Stripe every month. I would rather pay that time and keep the BSS than rent a prettier stack and lose the source. For what we actually pay (hosting + my hours, not a license), the value is there — if someone on the team will sit with the dates. If you only count the sticker price and ignore that learning, it looks “free” and then it is not.

Kill Bill is a performant, open source billing solution. It runs on Linux, which was a requirement. It integrates well with our application through REST endpoints while our application supplies the customer facing user experience. In addition, it is extensible through plugins. The developer community is active and responds quickly to questions and issues. The apis and the catalog and subscription management system are very robust. This provides us the necessary tools to layer on our own subscription and account management UX. The plugin apis provided the means for critical extensions for our niche market. The API documentation is also quite good. For any endpoint, all the necessary user information can be found there.