USA Bus Lines

Do you know a company that provides bus or motor coach service from Florence to Venice? I'm traveling to Italy with my family, and while we aren't so inexperienced to need the hand-holding of an organized tour, we would like to take a bus between major cities. My parents are older, so lugging our bags around busy train stations isn't the best option. I've identified a company that provides motor-coach service from Rome to Florence, but not one from Florence to Venice. Thanks!

What is a shag test in a bus? for an interview with a motor coach tour bus company