I assume you are referring to services like Super Shuttle. On these I tip a percentage basis that can vary widely based on the driver. I once had a driver who was excedingly polite and obviously took great pride in his van to the point of having magazines available for his customers to read; he got a very good tip, about 20%+ and in cash, not on my credit card. I have had drivers so rude that I would give them 10% on my credit card and try no to give a cent over that amount.