Optional Online Payment

Could we have it that we have an option for a patient to pay online when booking an appointment either via online booking OR via the App? At the moment, as I understand it, they either don’t have the ability to pay online OR they MUST pay at the point of booking. Could this work a little like Booking.com where you get asked to pay now or at the clinic? I know we can ask for a deposit but this isn’t the same.