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.