Almost everything can be booked on line nowadays, including bus, train, and plane tickets. There’s much less consistency, though, over whether you need to print out a copy of your ticket, except that my general rule of thumb that we only had to when we couldn’t find a printer easily turned out to be true depressingly often.
The worst offenders here are the budget airlines that charge you an additional fee for printing out boarding passes: Wingo hit you for $5 if you pay in advance and a massive $15 if you turn up at the airport without a printed boarding pass. Most of the bus companies in South America were rather ambiguous about it, implying that you probably could get away with having a PDF on your phone, but that it would be a much safer idea to print out the tickets just in case. Vietnam Trains get it right, and explicitly say on the tickets that you don’t need to print a copy (although the agents we booked through contradicted this in their covering email).
Boarding passes for the first three flights from Nairobi to Santiago - thankfully they didn't charge us to print these out
I wonder about the implications of this for the digital divide in general. When we lived in Singapore, it was easy to print out a few spare copies of our travel documents at home or in the office, but we often found it really difficult to find a printer we could use when travelling. Most hotel concierges will print out a few pages for you if you ask nicely, but if you’re staying in a hostel or AirBnB it can take hours to find a print shop or Internet cafe, and cost quite a bit per page.
The Mercado Benito Juárez in Oaxaca still has an Escritorio Público - a stall where an old man with a typewriter will write “anything from a love letter to an injunction” according to a New York Times article from 1985. In a not too dissimilar fashion, next to every government office in Cuba there are a few street-side stalls where somebody with a scanner, digital camera, and printer can help you sort out your documentation, or laminate the certificate that you’d just received. Maybe in the future we’ll see stalls in airports undercutting the airlines’ fees for printing out boarding passes.