Question for app developers: user data storage?

Hi everyone, I'm new to the forums but have a question that I'm hoping someone can help me with or at least point me in the right direction. I'm looking for the best methods to store user data from a game app and was specifically wondering: does anyone know if Game Center has any support related to this issue? The biggest issue for me is in-game upgrades (as in you download a game for free but then pay for additional content rather than buying the full version app). What would be the best way to preserve this user data so that the app does not "forget" that the user paid for the upgrade and asks them to buy it again?

My main concern is the flood of low ratings and bad reviews that come when users think they are being charged again for something they already paid for, even if it isn't actually happening.

I know there are a variety of solutions involving storage on the developer's end but I am wondering if anyone knows if Game Center or any other online networks support this kind of user data storage.

Thanks in advance for any help..