Of note, the same discount is available in-store via this printable coupon. (It's not valid at Barnes & Noble College Bookstores.)
If you pay $6 or more, you'll also get Knytt Underground for Windows (also compatible with Mac and Linux), NightSky for Windows (also compatible with Android, Mac, and Linux), and Really Big Sky for Windows, an even better deal. Your contribution can be donated to charity (Child's Play or the Electronic Frontier Foundation), the developers, or both. Deal ends December 12 at 1:59 pm ET.