U.S.P.S. delivers collected mail to its closest sortration plant. That plant then delivers all ex-U.S. mail to one of several very large sortation plants situated around the country. During 1986-1991 when I was an Operations Supervisor at Canada Post Corporation (C.P.C.) in Calgary, the nearest of these large U.S. sortation plants was at Salt Lake City, Utah. That meant that a letter sent from (e.g.;) Great Falls Montana (443 km./275 mi. south of Calgary) 1st went south 463 mi (745 km.) to Salt Lake City before being forwarded to Calgary.
Every Friday afternoon, the Calgary plant would get two wagons from Salt Lake City that contained between 80 & 100 boxes of mail of all types - letter, oversized letters & parcels. Depending on how busy it was at the time, either that Friday night or on Saturday, mail handlers in the Mechanical Sortation section that I worked would open those boxes & loosely sort the various types of mail into large wire "cages". The cage of parcels would be wheeled to the Parcels Sortation side of the plant. This meant that the mail may already have spent as much as 6 days in the U.S. while not only not getting to its final destination but, often, in getting farther from it.
The rest would be sorted into letters or oversized & wheeled to the appropriate machines in my section for sorting. All the letters that were subsequently found to have a hand-written address would be brought to the Manual Sortation section for sortation there. All that had typed addresses would be dealt with in my section.
A major problem is that people in the U.S. plant seemed ignorant of geography. Among the mail we received, a great deal (often more than 1/2) was addressed to other countries. It seemed they just dumped everything that wasn't a U.S. address in the boxes sent to us. We got mail meant to be in Mexico or places south, for Europe, Asia, Africa. While this mail was ultimately sent to its destination, between the up to 6 days that it "suntanned" while being moved about the U.S. & the time needed to get it sent where it was actually supposed to be, a lot of time was lost.