Google has created an open and easily implemented Google Payment API that web and app developers can use to allow shoppers to pay with their Google account. This means that checkouts that would normally require an account with the merchant to store payment information or require the customer to enter payment information manually can be handled by simply having Google hand over payment details that the customer already has saved in their Google profile. The API in its new, more user-friendly form is already freely available, and is being used by a number of early retail partners like Papa John's and StubHub. The API can be implemented with minimal additional coding, since it calls to and gets calls from Google on the server side. The biggest obstacle to widespread implementation at the moment is the fact that only four major payment providers are on board at the moment, but Google has promised that this situation will improve, with ten new providers already slated to come on board in the near future.
On the user end, the Google Payment API can be used to produce a Pay With Google button, as seen in the GIF below. This particular implementation is sampled from the Instacart app. When tapped or clicked, this button simply asks a user to double-check that they're logged into the right account, then choose which card on file they want to pay with. From there, handing over payment information and making confirmations is all handled automatically, and the user lands at the post-order confirmation page. Naturally, the API can be customized if a developer wishes, though the extent to which a developer can change the user interface of the API and other details is a bit unclear at this point, and will likely change over time.
The new Google Payment API promises to make shopping that much faster and easier on the web, and seems to be easier to implement on the vendor end than the web and app versions of Android Pay. It also has the benefit of being server-side, and thus universally available. Whether a customer is using Firefox on a Linux PC, Safari on a Mac, or Chrome on an Android smartphone, this API should enable them to pay by simply logging into their Google account and picking which linked credit or debit card to use.