@evanm I would recommend using a JWT in your API and storing the user id to associate the user to your stripe user account.
You can find more about this in our tokens docs, here you will want the sub.
https://fusionauth.io/docs/lifecycle/authenticate-users/oauth/tokens
On point two it is a classic it depends scenario. This could be handled by logic that you build, or if it makes sense you could use FusionAuth's groups or entities