I had a similar situation where I absolutely lowered my tip. Asked my hotel in south NJ to book me an early AM car service to the office. Specified destination (just the name of the town...I didn't have the address when I made the request the night before) and pickup time. Even discussed with FD how long to allow with traffic.

Next morning, about 20 min before my scheduled time, FD calls my room and says the car is there. Great, I reply that I'll be down in a few as I'm still getting dressed. Five mins later they call again to say the driver is waiting.

I arrive downstairs at 7:10 for my 7:15 pickup and am quickly ushered out the front door by the FD person and driver. Didn't even get a chance to check out and get my folio. When I climb in the back of the SUV, another woman is sitting there.

As we are driving, we discover that she had arranged a 7am pickup to go to the airport. My office is sort of on the way but requires a 10-15 minute detour vs. taking the highway straight to the airport. It is clear that she also was unaware that this was a shared ride service.

I got a direct ride to my office, but I did adjust my tip substantially because it felt like the driver was double dipping his fare (mine was certainly the usual full rate). I also felt bad that the woman was now cutting it close for her int'l flight.

And the last kicker: never got the hotel folio emailed and had to spend another couple days getting SPG to nag the property to send it...