SuperShuttle had no offer of a carseat when I used them. I had my own for DS. I can't even fathom having DS in a taxi or shuttle without it.
Though since I grew up on Greyhound buses (dad was a driver), and we take transit around town, buses are a whole different thing, and I have no problem with a bus and of course no carseat (no seatbelts).
But taxi or shuttle, kiddo's in a carseat all the way. Not a huge deal, either. We even did the carseat thing for a 10 minute taxi ride up in Vancouver...just takes a minute of wrangling and you're good to go (assuming the person who will be doing it in the taxi/shuttle is the one who knows how to do it!).
My supershuttle driver from Ontario airport 2 weeks ago seemed very happy that I had a carseat and knew how to install it, though he was also more than happy to take it out for me when DS had fallen asleep just before we got to the hotel, and the belt got stuck in the path while I tried to quickly uninstall it. Nice fellow!