What is the customary way of tipping a car service??

Can anyone tell me if its customary to tip a driver on the initial trip to port, on the return trip home after the cruise, OR tip both ways? AND if you do tip both ways are you to split the 20% in half each way? Funny how ya never have to think about these things until they come up. Thanks!

You tip in each direction. No guarantee it will be the same driver both ways.

I agree to tip in both directions--15-20% of the one way fare.