Shuttle drivers will generally get a couple bucks from me if they help with the bags, and generally nothing if they don't. I did have one stop for me and let me on the bus the other day, and even though I had no bags that day I gave him a couple bucks.

I did have a rental car shuttle driver in Galveston that went out of his way, after his official hours were over that day to help me (we'd returned the car after the office was closed, and were standing waiting for a cab to the port to get us, which was running much later than it should have been. Shuttle driver was returning to park for the day and offered us a ride over to the port. It was after a long series of flight problems and lost bags, and we were genuinely concerned about actually making it to the ship before it left that day. He got a $20 for that, and my undying appreciation (which probably doesn't really mean a whole lot to him)).