subtracting from multiple rows in a cvs file in python

I am trying to figure out how to subtract a set amount from a row if it goes over a value. For example, if I have a list of library accounts and I need to write a function whereby if an individual goes over their printing credits, they are charged an additional 0.10p. At the moment I have my columns with the library IDs and one with their account balance and limit they are allowed to go over. However, I'm not sure how to write a function that allows me to see when their account balance goes over the limit whereby I charge 0.10p. Any help would be appreciated.