The documentation comment on the PaymentsClient.isReadyToPay(request:) method lists the following check as one of the minimum requirements to finish a transaction using the Google Pay API:

User either has or can add a card in flow according to the specifications given in the IsReadyToPayRequest.

It's not entirely clear what "can add a card in flow" means and it's not entirely clear what parts of IsReadyToPayRequest determine whether a user "can add a card in flow".

My guess is that (1) "can add a card in flow" means the Google Pay card selection sheet will show a button that allows the user to add a card to their account, and (2) setting the IsReadyToPayRequest.existingPaymentMethodRequired property to false is what communicates to Google Pay that a user "can add a card in flow".

Assuming the above two guesses are correct (please correct me if they're not