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. Credit card information is not stored within MassageBook.