How do I be polite to a Gamestop Cashier?

A cashier/manager at Gamestop assumed that I wanted to preorder MW3 when I preordered Assassins Creed Revelations and he put down 5 dollars on MW3. It would have been easy if only that had happened but he gave me a reward card membership (which is 15 dollars) for free and he also he gave me full return value on inFAMOUS 2 even though I used it and I couldn't return it for full value. And then he used that credit to put 45 on Assassins Creed and 5 on MW3. When I come in to Gamestop again to pay off Assassins Creed, how do I ask to transfer the store credit from MW3 to Assassins Creed without seeming like an ungrateful jerk?