I never tip shuttle bus drivers. Even if they pick up my luggage, that's what, five seconds?

A porter who carries your luggage all the way from the curb, to check-in, up the elevator to your hotel room and finally inside the room should be tipped, but a shuttle bus driver? That is a hand-out, unless you ratio it for the time involved, which would be a nickel a bag. Tipping a nickel appears to be an insult, so make it zero.

I've seen people tip a dollar to the shuttle bus driver when they had no luggage at all other than a carry-on. Why???