I feel the USPS should just handle letters, flats and parcels shipped by individuals like eBay sellers and private individuals not commercial retailers like Amazon, Target, Walmart and such. They subsidize that low cost rate with money they make of First Class and Priority Mail.
They are in bad shape and have been since the start of the computer age of email, electronic statements and e-banking. First Class mail has dropped dramatically
As for the trucks leaving empty from sorting centers I’m sure UPS and Fedex do the same thing. That truck is scheduled to be at the next location to be loaded and go out from there. If that truck doesn’t get there, then the next load is delayed. I know we get 4 trailers in and barely fill a half of a 28’ going out. We’d have lots of trailers on our lot if they didn’t haul the empty ones out.