Interesting post for a first time poster! I tip my DME driver 'if' there is something special. I refuse to tip when I'm told that they accept gratuities. That's pretty much the same thing as asking for it. My dd makes minimum wage and no one is tipping her, even when she does something out of the ordinary and makes the guest very happy!
I seldom have a bag that the driver touches, so I'm not happy to think that the driver expects a tip for driving the bus! There is signage that lets everyone jnow that 'gratuities are always gratefully accepted for exceptional service'. I seldom get exceptional service.