There is a major glitch live now in the Microsoft store. The glitch enables you to increase the quantity of the Assassin's Creed 2 and Dragon Age Origins and not alter the price. Since the two games are already free with the purchase of a qualifying Xbox 360; someone managing the site may have missed the checkout glitch that allows you to increase the quantity without increasing the purchase price(For the games only). Notice the difference between the pictures? The price of the Xbox 360 will increase the subtotal of the purchase upon quantity change.

To test it yourself while it last:
Add a qualifying Xbox 360 to your cart
Add Assassin's Creed 2 & Dragon Age Origins to your cart
Change the quantity of the games
Notice the subtotal remaining stationary

Source: ironstarmovement