It's important to reserve your U.S. clinical experience quickly so that it is not taken by another trainee.
If you select the "Book Later" option, then you will be entered into the general AMO application process. Your application will be reviewed within 24 business hours, and upon approval, you will be eligible to book by submitting the down payment.
With the book later option, you do risk losing the program seat to another applicant. There is no guarantee that the experience seat is held unless a booking is made by submitting the down payment.