What veichle are you using, Seadan, SUV, or Van? Are you making the grocery stop? Under most conditions I would tip 20% of the bill for a seadan, less for the SUV or Van.
EDIT: I don't know, maybe I am thinking more along the lines of different types of transportation. ie. Taxi, Shared Van, etc Looking at this company it seems all veichles should be tipped the same.