The subscription setup in FoxyCart only handles the quota_id - it's not a cart based setup that can have other items placed in there, and will only handle a single subscription at a time. Doing a-la-cart pricing via a subscription is not going to work with PayPal (as far as I know) - the price can't be changed on a monthly basis without the user canceling and restarting the subscription - i.e. once the subscription price is set, it is "locked in" on the subscription record that is maintained by PayPal and referenced by FoxyCart when it goes to make the monthly payment.
For what you want to do, I think you're going to need to use Stripe directly and basically do "touch" billing - i.e. save the customer stripe ID to the user's datastore, then create a custom module that on a monthly basis figures out how much each user owes, then charge their credit card that amount by initiating a new charge to their stripe customer ID.
This is considerably more complicated than the FoyxCart module is setup to handle though, so it's something you'd need to build.
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Brian Johnson
Founder and Lead Developer - Jamroom
https://www.jamroom.net