Tipping is entirely up to you. If the driver stows anything for you, he/she will expect a tip....they are handling baggage for you after all. BUT....I have had a driver stow a largish carry on bag, and was prepared to tip upon arrival at my resort. However, that driver stood up, at each resort stop and announced that gratuities are gratefully accepted for exceptional service. Ok....there is signage, right above the windshield, that states the same thing. I'm not crazy about basically being asked for a tip. So, my tip money stayed in my pocket.
However, I had one ride, with nothing stowed beneath the bus, that had an incredible driver. He was funny, told great stories about the area, drove very safely. He really added to the start of my vacation....he got a tip! I think he was really surprised!
When I tip, it is usually $1-2 per bag.