The sorting centers are completely different, since the methods of backbone transit are separate. Express only goes via airports (except for regional city pairs

The sorting centers are completely different, since the methods of backbone transit are separate. Express only goes via airports (except for regional city pairs like L.A. to San Francisco). Home/Ground only goes by road/train, so you'd have to consolidate somewhere in between. Also, a significant fraction of the Express traffic is stuff that is time-sensitive delivery. Things like getting signed paperwork so people can buy houses. They really hate delays. In fact, if you spring for the "priority overnight" they'll send a driver to your door with a blank waybill and envelope, and wait while you fill it out. It's quite a different service than Ground/Home.