There are many stands right at port exit selling tickets. No need to buy in advance. Can pay with credit card. There's a promotion for City Sightseeing Hop on Hop off bus (EUR15 only). City Sightseeing offers a free shuttle bus to stop no.1. City Sightseeing HOHO has lousy commentary. No extra info provided. Very old worn out seats. There's actually nothing much to see. You can just walk from port, straight up the road, Via Settimo (shopping street), Via Maqueda (pedestrian street), to the Cathedral and back.
Visited: Dec 18, 2017