There is no standard written for extra service. The driver has to earn that tip and many don't. If you get a driver that tells you to come downstairs to a parking number, load your bags into the trunk and sits like a bump on a tree I wouldn't tip him a dime. Now on the other hand you get a driver who busts his but to make you happy than you can determine what you want to give him and maybe he would deserve 20%.
Many companies request a 20% gratuity because that is a way that they can pay their drivers.