Heinrich von Stephan spent years talking countries into one postal rulebook, and in 1874 twenty-two of them sat down in Bern and signed. Before that a letter abroad could be charged by every country it crossed (rates worked out at the desk, badly), so plenty of people just handed it to a friend going that way. Bahawalpur put the Bern monument on a stamp — robed figures round a globe, passing letters along.
Once the rates were settled the timetables had to hold up their end — mail steamers sailed to a published hour and the night mails were built to meet them, so a bag could leave one country in the dark and make a connection in another before breakfast. Signalmen knew the mail by the time, not the engine.
The bit that still charms me is the international reply coupon, which came a bit later — you buy one here, slip it in the envelope, and the stranger at the other end swaps it for the stamp to answer you. Prepaying somebody else's reply across a border; a very Bern idea.