If a contact has card/payment details stored on their profile, you can process a patient's payments using one of their contact's card details instead of the patient's card details. This allows you process payments for a patient using a contact's card without billing the contact directly on the invoice.
To do this, you must enable card sharing for the specific patient-contact relationship when adding/editing a contact for a patient. Card sharing applies only to the specific patient-contact relationship; if you want multiple patients to be able to use the contact's card to pay, you must enable it for each patient-contact relationship separately:
Once card sharing is enabled, you can select the card you want to use whenever you process a payment: