just did mine, it's a snap, make sure your already logged into your amazon account, makes things so easy that way.

Amazon is offering a $10 Kindle Textbook Credit. Just enter the code ETXTBOOK. This offer is valid from December 30, 2011, through January 9, 2012, and subject to change. You must redeem the credit by 11:59 p.m. PST on January 9, 2012. Limit one credit per customer.