By default when you purchase for a customer, it will store the credit card on file. Any default payment method will be automatically selected as the first payment of choice when purchasing an item on point of sale.
If you need to add a card for whatever reason you can do the following
Go to client profile
Under payment cards tap the + to add new
3. Add new card and click "Add a new payment card"
How to delete a payment method
How to set a default payment method
