I assume you're staying at an onsite hotel? When you check into the hotel, they'll give you a slip of paper that you can use to charge items to your room. You must have this paper with you to do so, so carry it with you, and of course be careful with it!
I think the only things we charged to the room were character meals. I read that charging purchases from the shops is a bit more of a hassle than it's worth, so we just used a credit card for everything else.
You can also have purchases delivered to the hotel lobby, but they will not be available for you to pick up until the next morning. Again, it may be more of a hassle than if you just hit the shops on your way out of the park, particularly if you leave in the middle of the day.