You should probably check this: https://support.google.com/googleplay/contact/nexus_price

Ha, wish I had seen this link first. My Nexus 4 arrived last Thursday so I was well within the 15 day window. I just called their customer service (https://support.google.com/googleplay/answer/3053039) and they processed a refund for the difference. They're a bit swamped right now so you may have to hang on for 10-15 minutes before you can talk to someone.

Thank you, I didn't know about this. Very helpful.

If you have not yet received the shipment, you can instruct UPS to refuse it and you will be refunded everything (incl. shipping costs)