A Client can only pay for the cost of the Service when prepaying at the end of online booking. A tip cannot be added during that process.
A tip can only be recorded when checking out an appointment on the Schedule. If a client tips after the session, you can add it when checking out the prepaid appointment.
Note: Even if the Client tips with the same credit card they used to prepay, the card has to be charged and processed again in order to receive the tip payment.