PaymentType indicates the form of payment used for either a Google-provided or Action-provided payment method. The action must also provide a display name for the payment method.

PAYMENT_TYPE_UNSPECIFIED Unspecified payment type.
PAYMENT_CARD A credit/debit or prepaid card. At present, all Google-provided payment instruments will be of the PAYMENT_CARD variety.
BANK Payment method is a bank account. Action-provided only.
LOYALTY_PROGRAM Payment method is a loyalty program. Action-provided only.
ON_FULFILLMENT Payment does not occur until fulfillment of the order, such as cash-on-delivery orders or payment at time of service. Action-provided only.
GIFT_CARD Payment method is a gift card. Action-provided only.