Both Amazon and Greenman Gaming are offering incentives for preordering the game. Amazon is offering
10% off plus a $10 credit plus a free mission DLC. GMG is offering
10% off plus either $8 cash back or a $12 GMG credit. They also have a tiered reward system depending on the number of preorders. The first bonus (already unlocked) is the same DLC as Amazon, the second tier is Spec Ops : The Line for free, and the third is XCOM : Enemy Unknown, its DLC, plus the four classic XCom games.