I worked loading and unloading at a FedEx ground sorting facility for a couple weeks 15 years ago; some of the trailers had rollers down the middle to speed loading and unloading. For the others we had rollers to put into the truck. But you wouldn't want the whole bed to be a conveyor, the sorting really needs one package at a time in the line, otherwise it's hard to scan and direct packages, and packages will get jammed at the turns.