When you book, the payment system holds the amount of your booking fee from your credit card. This is a guarantee to assure that the fee can be charged later. The payment system charges the booking fee to your credit card on your check-out date. From the moment you book, the payment system checks your credit card availability by the time you check-out and sends you a message ; the payment has been successfully confirmed.
If you have any doubt about this matter, please contact your credit card company or your bank for further information.