Many parks offer this great service. One thing I was surprised at in June when I visited Disneyland and California Adventures was the fact that you had to take the package to the front of the park yourself. Why don't they offer pick-up from the store? I know Dollywood has offered "Package Pick-up" for many years now. When you purchase any item at a store you leave the package with them fill out two slips of paper and take one and claim your package at the front of the park on your way out right next to the turnstiles. Usually during the summer (when they close at 8) the last pick-up is around 5 to 6. Its a great benefit because you don't have to physically take your package all the way to the front of the park. You just purchase and the park goes around and pick up the packages three times each day from the stores and its waiting for you in a dry climate controlled building near the exit. Very convenient!
I once forgot something that I had delivered to the front of the park at SeaWorld before and they mailed it to me for free.