paymentMethodType
A string representation of the customer's desired payment method:
If this is a Stripe payment method, see https://stripe.com/docs/api/payment_methods/object#payment_method_object-type for possible values.
If this is an external payment method, see https://stripe.com/docs/payments/external-payment-methods?platform=ios#available-external-payment-methods for possible values.
If this is Google Pay, the value is "google_pay".