When I used to drive for Super Shuttle, I did this all the time for any customer who asked. Many would say to me all I have is a $$, can you give me $ back or Can you break this $$ bill, so I can tip you and also the Porter at the airport.

I wasn't the type of person who asked for tips, as it is impolite. However, many drivers would have signs in their vans that specifically said gratuities not included in multiple languages. I found those that did had more service complaints to the company.