I'm building boathouse.pro which offers Add-Ons for Paddle Billing (that's a SaaS subscription billing provider that takes care of taxes which I'm betting on).
Currently I already have customer portals, embeddable pricing tables with automatic currency conversions and cancellation insight forms. If everything goes well this week I'll have some cool new email features.
Paddle is a payment provider. You may have heard of Stripe, Paddle does similar things but in a different way (what they call a Merchant of Record, so basically they just handle a lot of stuff on your behalf, like sales tax).
Paddle Billing is their newest platform (they call the old one Paddle Classic now) and it's just a big upgrade basically. Unfortunately it also meant all the people building software on top of them like my project had to implement everything from scratch - but I personally like the new APIs from a developer perspective, so happy they did.
See here: www.paddle.com