I take car services multiple times per week, at home or on the road. Over the years, I've used various providers in the NY/CT area, as well as Boston Coach and Carey when on the road, including abroad. In every single instance the car service costs billed to my employer already include a tip, broken out in an itemized accounting of costs.

So, it's no different than when a restaurant adds an automatic 15% (or 18%, or 20%, or whatever) service for large parties, or your hotel room service bill has a required % service charge, or when a resort does the same to your room bill. There is no expectation or need for additional tip, that is the point of those charges. So if you're one of those people that still likes to tip extra in those situations, then do so. If you're not, then don't tip the driver any extra either. They are getting their tip already.

My employer uses a car service for runs to various airports. The amount they bill the company includes a mandated gratuity so I would never dream of handing more over.

I agree that a $10-20 is the way to go. However, for me at least, I like a quiet ride and if I get the drivers who won't take the hint to stop talking, I may reduce the tip.

I think it’s better to err on the side of caution. If I am ever doubtful about tipping for a service and don’t have anyone to consult, I usually tip the person and make it a point to find out from a reliable source what I should do if a similar situation presents itself in the future.

I always receive the car service..Thaks!

I do not tip a car service unless it was explicit that gratuity is not included. Car services are not regulated like a taxi and they are free to charge reasonable rates to pay their employees. You are already paying a premium above a taxi so if they felt that the ride should be 110 instead of 90 they would charge accordingly.
If I am booking the car service I will be sure to ask, does this cover ALL costs what is my final bill, they will then say if they expect a tip or not.
If they went above and beyond what was expected I would tip to show I appreciated the effort.

The general rule I follow is the tip is paid when the transfer service is paid. If its prepaid, then I would assume the gratuity is prepaid. I would usually pay in the 15% to 20% range on exceptional service, although it is on the generous range.