If the customer has already booked something with you, they'll have a card on file in their account. You can go in and edit/remove or add a new one to that customer.
How to add/edit/remove a credit card on a customer's account:
Click Customers.
Click on your customer's name on the list.
Click the pencil icon to add, edit, or remove the card number.
Note: Currently, you can't have 2 cards for one customer.