When you book, you pay the deposit your beauty pro sets, and nothing more. There is no booking fee: FeelPretty charges you nothing for using it. If there is no deposit, you pay nothing to book.
On the day, you pay the rest of the price to your beauty pro, in a way they accept. Your booking shows how much is left.
Some beauty pros let you pay the rest in the app, so you do not need cash. See Paying the rest in the app.
Prices are set by your beauty pro and you always see the full price before you book.