Why did Kevin and Paul give the clerk girl 20 bucks, i thought it was strange they gave the girl 20 bucks the book could not have cost that much money lol ?

They wanted to get out of there as quickly as possible and were embarrassed to show the clerk they purchased the book "Everything You Always Wanted To Know About S** But Were Afraid To Ask". The book was $7.95, so it was not stealing with them being owed $12.05 in change.

Of course, Kevin's parents had that same book, too!