I sometimes receive car service from the airport to my hotel. I get invited to give talks, and the people sponsoring these talks will hire car service for me. These are the classic, black, Lincoln town-car type deals with a driver and no meter, etc.

When I leave the car, there is no transaction of money; the car is already paid for. However, I am not sure if I am supposed to tip those drivers or is it included in the total fare? I know that regular metered taxi's one gives a tip, but what about when one receives paid car service?