When I use a car service, the driver usually takes my luggage at baggage claim...I usually have to argue that I can take a bag, there isn't any need for him to take them all. Then, he loads the bags into the trunk. When we get to the resort, he takes the bags out and hands them over to me. He gets a tip at this point.
If I get a driver that expects me to lug my own bags and put them into the trunk myself, then he is not getting a tip. Luggage handling is part of the service.