DisneyZB,
I have no personal experience with delivered groceries but I have read the answer on here often enough. The answer is early check-in is not required. Everyone has a pre-assigned campsite or cabin and if the groceries are delivered before you arrive for a cabin stay, Disney will give the delivery person the cabin number and the groceries have been placed in the cabin refrigerator in the past. That cabin is held for you until you arrive.
Bama Ed