In the middle it links to the post he made on the 25th anniversary, which talks about how they would ship the gold masters to Europe. He says they didn't have time in the schedule to mail it, so they would go to the airport, find a flight to London, walk up to the gate and find a passenger and ask them to carry a pack of disks with them, and tell them someone at the other end would meet them at the gate to pick it up…
My dad used to do the same thing all the time but with trains and coaches - if you had to send something to the other side of the country pronto, you'd go to the train/coach station, speak to the driver, hand them some money, and they'd happily take whatever you were sending with them. It was literally the fastest way to get something somewhere. It doesn't surprise me at all that people used to do the same thing with…
Seems to work really well, from the POV of the driver, the sender, and the recipient. Cheaper than DHL, more trustworthy than the post office, faster than both!
It's routine enough I can't imagine the bus service doesn't know about it, my bet is they tolerate it because it lets the drivers make a living wage (and thus gets you better drivers) and because most packages aren't big enough to be disruptive.