I bought it from the Google Play store last year, but I don't know how this warranty thing works. I don't even remember if I have the original receipt :\
Ok I had to return a Nexus 4 for a swap when my volume Key broke this is how it went down
1. Google asks for your Google Account and from that they find your address and purchase history no recipt needed.
2. They charge current price of the phone on a temp charge to your Wallet Account
3. They overnight the phone
4. You swap phone in box with broke one
5.Send it back
6. When they recipe it they take the charge off
7. You go about your business
It took overall about 5 days no fuss no muss.