Everything comes to Costco stores in pallets, they don't do shipping boxes.
With the exception of certain times (e.g. Christmas) or certain books (e.g. Spare), they don't sell enough of any given book to have a pallet in the store for that book.
Mixing items/books on a pallet is a dumb idea. With single product pallets, you replace the pallet once its empty with another pallet. If you have two items in a pallet with one sold out and another barely sold any, what do you do?