For those that purchased the device before 10/25, make sure to redeem the code as It's only good through the end of the year and the credit needs to be used before the end of 2016. Also, if you don't redeem the code yourself you can send the one-time code to a friend or family member to redeem.
I have a feeling I'm going to go on an app spending spree sometime in late 2016 before this credit expires. With Google opinion rewards, the money I have spent has been replenished.