Using the get-payment-methods endpoint will allow you to query payment methods used prior by a guest. The returned payment methods can be chosen from by the user. The one they use should have it's id sent along with the checkout request.
|id||string||id of the payment method|
|last4||string||last 4 digits of the card number|
|exp_month||string||month of expiration date|
|exp_year||string||year of expiration date|
|brand||string||Brand of the card|
|default||boolean||This is true if it is the guest's saved default card.|