Many years ago we did the John Day River, four cars to be shuttled total. When we got to the takeout, one of our cars was missing. We eventually were able to contact the shuttle people and to make a very long story shorter, it turns out that a different shuttle company had picked up the wrong car at the takeout, same car make, color, and key hidden in the same spot, and had taken it to their storage area. It was a long afternoon, but eventually we got our car back. Our shuttle company refunded our entire shuttle bill because of the mix up, even though it wasn't their fault. We ended up mailing back 50% of the shuttle fee, as none of our group felt that they should take the hit for the situation.
All these years later, should we have just sent them the entire shuttle fee anyway? Hard to say, as the shuttle company was so adamant about refunding us for our troubles.
Then there is the story of getting to the takeout at Carey Creek, and finding a vehicle with a destroyed tire, and the donut spare flat. But at least it is mounted on the car. But getting back to the original question regarding a tip. Yes I always feel like I'm very generous because I want them to take care of my vehicle.