It's break bulk shipping.
Regional terminals (post offices) build up Truck loads going to various areas of the country. If the area your package is bound for didn't generate enough tonnage that day, the package will go to another terminal in the region that has generated enough tonnage to move trailer into your area.
Trucking Companies have been doing this for decades. Train companies for over a century. It's nothing new, and with regard to the use of manpower, fuel, and equipment, it's actually more efficient than running packages, or even partially filled trailers (LTL- Less then truckload) shipments to a given area.
It might take a little longer, but it is more cost efficient, and when an organization like the Post Office, that is losing Millions of dollars a day, can find a way to increase efficiency. well, that's what they are going to do. Even if it costs you an extra day or so in transit.
