https://www.theverge.com/2018/5/17/17364586/google-store-return-gifts-credit

The Google Store now lets customers return gifts, so if you’ve been holding on to a Google product like a Home speaker or an extra phone case because you’re not sure what to do with it, you can now return it for Google Store credit. To return the item, just go to the gift return page. You can sign in to your Google Account to autofill some information, or just provide your name, order number, and ZIP code. Once your order has been found, pick the items you wish to return, and submit those selections to Google. You’ll then be emailed return shipping labels for you to post with the items. Once Google receives the package, it will credit your account in the amount of the original purchase cost of those items.

Google says the credit you receive from the gifts you’ve returned won’t expire, will be non-transferable, and will only work on devices sold in the Google Store.