[h00dui]

Hard to be very detailed here without knowing a lot about what you are doing.. However here are some ideas.

1. USA - Stripe for payments and managing subscriptions. They have numerous tools for doing so. Their subscription handling is pretty good at the moment and useful for people who want to cancel etc. The handled processing, fraud etc. Pretty simple to setup with your saas.

2. Talk with an accountant or lawyer for your specifics regarding the state. They differ wildly so you want to make sure you are prepped and handling that from the beginning. Any good accountant should take about an hour to break this all down for you. Worth the $$.

Best of luck.