I always send a box of "magic" down in advance.
Simply address the box to your attention c/o resort name and address. I always put the arrival date and confirmation # on the box as well.
You may want to consider UPS or FexEx. You will receive a tracking number so you can make sure it arrives in advance.
They will store this for you and a note will pop up for the castmember when you are checking in. Sometimes I pick it up....generally I have bell services deliver it to my room.
This is not YOUR case.....but be advised......you will be charged for this service if you stay at a Disney hotel with convention facilies.....based on the weight.