Booking online

Hi Chai!
Every single time I hired a bus in Japan for anything, I didn't need to print out anything.
They usually have your name on a list and generally, you just need to say that you are there or have your ID to prove that you are you.. but you probably won't need it.

Also, you could have a screenshot of the confirmation email on your phone.
papers are a thing of the past, you can show it on your phone.

The only cases that I have seen, in which you need to exchange the printout for a real ticket is when you buy tickets for music concerts and soometimes for cinema.

I'd say, keep a screenshot of the email on your phone just in case and go for it, you most probably won't have any problems. That's what I'd do, because the bus driver usually just have a list of the passenger names.