This is more low-level than that. This is an API to retrieve payment details, not to process an actual payment. You'll want to use Stripe or the like for that.
It is for app and web site developers that want to accept credit cards through the new Payment Request API. The backend would process the credit cards with their usual payment processor. The payment API is easier for users since Google remembers credit card details and they don't have to type them into a form.