
If you have enabled the booking widget for existing clients, you'll see a checkbox within the Services tab that allows you to indicate that the template should only apply to existing clients.
Upon checking that button, you'll see that you can now decide how the system should source the services -- you can either enter in specific services and fees OR the system can utilize the client's last billing info (e.g., if the client was last billed for 90834 at $200, then when they utilize this template to create/reschedule an appointment, they will be scheduled for 90834 at $200.)