You may add a new card when booking a spot on the Order Details page, or change the payment used for that transaction. You may also add a card and the default payment method, from the menu, which changes if you are on the web or mobile versions:

Adding a new card on the website:

Click ‘My Profile’ in the menu, then ‘Payment Information’. 

Adding a new card on the mobile app:

Tap ‘Settings’ in the menu, then ‘Payment Settings’.