Do they allow apps they extend Stripe's functionality but charge for it?
"Transparent and consistent pricing You must clearly state your app pricing up front, without hidden costs or fees. App pricing must also be consistent with off-marketplace prices."
https://stripe.com/docs/stripe-apps/review-requirements#app-...
Though I suppose Stripe is watching for apps doing things Stripe should/could do itself as a real-time "what would people pay extra for?" list.