Polar

Polar is a hosted billing platform. It handles usage-based, subscription, seat and credit pricing, checkouts and webhooks for AI products and other software. A site or app connects to it through the public API, the webhook API and framework adapters, and the README names JavaScript and Python SDKs. The README describes Polar as the merchant of record, which takes on receipts, customer accounts and sales tax or VAT. The documentation covers the API and the integrations.

Worth knowing

Polar is sold as a hosted service. The docs say self-hosting is technically possible and recommend the hosted service instead. Organizations have Owner, Admin and Member roles, and single sign-on works with any OpenID Connect provider.