After you click Get Deal scroll down the page at GameChanger. Donate the amount you want ($1 is all you need to get a copy of Tomb Raider). Scroll down further to enter payment.
Donating $20 or more also gets you entered into 2 prize drawings with a chance to win a Collector's Edition of Rise of the Tomb Raider & swag bag or XBOX One console + games. Donating $50 or more and get a personalized thank you letter.
More info about the GameChanger charity here.
The Game of the Year Edition includes: