There are 2 ways to create a booking/trip.

Option 1 (Create invoice & Booking) : At the time of creating invoice, there is an option to convert the invoice to booking/trip. By doing this way, The invoice will be converted as Booking/trip without a customer payment.

Option 2 (Convert invoice to Booking) : you can update payment on invoice, When you are updating the first payment on invoice, the invoice will be converted to Booking/Trip. The invoice will be moved out of invoice section and placed into Trips section.