I've got a couple of timetables for the bus services to Arabba (for
the Giro's garnfondo even at the end of the month).

Has anyone used these bus services before? Would I need to pre-book or
an I just pay at the bus station in Belluno, where I plan to catch it?

I'm planning to do an overnight stop-over in Bulluno because the train
journey up from Cesenatico/Rimini doesn't get me to any of the towns
on the Arabba bus routes early enough to make a through connection in
a single day.