I'm designing an iPhone app that allows a user to book into a class. The user has the option to pay immediately for the class, or pay in cash upon arrival at the physical location.
What is the most seamless way to let the user have this choice? Ideally, they would pay immediately, but given the constraints (and not wanting to ask the user every time they click book) how could this be best achieved.
Further Info: The book button currently occupies 60% of the tab bar, and the price field the other 40%. The user will also have the option to add or change payment method, or purchase a subscription when they click the price field. When the user first tries to book a class, they will be prompted to add a payment method or purchase a subscription