The Google Play Store credit that comes with the Nexus 7 has a dwindling life-span. As a reminder, Google's Play Support site shared that the $25 dollar credit that comes with the Nexus 7 to purchase Google Play content, (like movies, books, games and apps), will no longer be offered with the nexus 7 if you purchase one after September 30th, 2012. Here's a quote with some of the details,
Source: Google Play Store SupportEligibility
The $25 Google Play credit offer ends September 30, 2012.
To be eligible for the $25 Google Play credit on Nexus 7, you must have:
A valid Google account; and
A valid credit card stored on your Google Wallet account Please be aware that you will only receive one $25 credit per Nexus 7, credit card or Google account. If you did not receive the credit, you may not be eligible to receive the credit.
At this time, virtual credit cards are not accepted for the Google Play credit redemption.
Terms and Expiration
Offer ends September 30, 2012. Some exclusions apply. Value subject to expiration. Read the Google Play promotion terms.