Flint is your app and Stripe is your payment processor. Stripe’s transaction fees apply to each transaction. They are 2.9% + 30¢ per successful charge. Stripe has no monthly fees, no refund costs, no setup fees, no monthly fees and no card storage fees.
For more details see Stripe’s fees page: https://stripe.com/us/pricing
Stripe also has no monthly limits. You can therefore transact as much and as frequently as your business requires.