Zapbox itself might be a little bit bare; but a simple NIP for some receipt such that regardless of payment method, publishers could make it clear to the relays used what npubs are supposed to receive what should cover most usecases (including tiers and whatever).
The 'badge' is a nice idea, could indeed be a simple tag inside the event.
Boom, we just fixed substack, podcast subscriptions and whatever else.
Now im busy figuring out the spec for kidstr/TEPP (trust extended permission protocol), so someone else picks this up including a decent podcast standard and we can kiss RSS goodbye.