Unfortunately, you are not addressing Apple employees here - we are just other users like you and can't help with such issues. You need to address the issue with the store. They should have a copy of the sales slip - you should get a copy of your receipt. You have proof of your cancelation? Use both when talking to the store.
Contact Sprint. Since they are the ones that charged your card, they are the ones you need to bring it up with. If you still have trouble, contact your credit card company to dispute the charges.
Best of luck.