I drove a car rental shuttle for a while many years ago. Most customers don't tip, maybe 30-40% did. I helped with luggage (except when it was obvious that the customer didn't want help), directions, etc. I never expected tips, but certainly appreciated them. I also never felt badly towards customers who didn't tip. The job paid minimum wage, and I went in expecting to collect enough tips to make about 1.5 times min wage overall, and that is about what the average was. So if you want to give a buck or two, go ahead, the driver will appreciate it. If not, just ignore the guilt trips from the "tip everything that walks" crowd here.