You are charged when you complete the booking at checkout. If you chose to pay later (e.g. hold), you will be charged before or at the time of the appointment according to the provider's settings. For additional charges (add-ons or products added during the visit), you pay when you tap Pay on the booking detail—either in the app (which may open the payment page in the browser) or on the web.
If payment fails (e.g. card declined or insufficient funds), you will see an error and can try again with the same or another payment method. Update your card in account settings if needed. Until payment succeeds, the booking or additional charge may remain unpaid; the provider may send a payment link or you can pay from the booking detail.