I probably overpack for Apple returns or private sales and trades, and haven't had an issue. When the shipping is paid by Apple or another vendor, the box size usually doesn't matter. I always keep the original boxes, surround them with a couple inches of bubble wrap, pack them in a larger box, and fill in all the spaces with more bubble wrap and other filling. And I take photos of the device, serial numbers on the boxes, and the whole packing process. I realize this is probably overkill but I'd want to be sure if there was ever a dispute.