Question on Limos

Okay so I think the enxt thin we're going to book is the Limo. I just called a Vendor from here and the price they gave just seemed lower than all the other places. Also, one thing I just cant understand is the 20% tip.....does anyone know if that is the standard or can we change it based on the service?
Also, does anyone have a 'list' of question they asked when they booked their limos?

Thats all the questions I have for now on Limos. Thanks ladies for your feedback!!!

Well i booked with Royalty.....they included the tip in the price but im sure you give extra on the day of.......