it is in our contracts, however we have tried to not charge the fee and give people just a little extra time to pay. 85% of people ended up paying, but i have a few deadbeats. We started with a simple "hey, um, you owe us money" style letter, than 3 formal collection letters with late fees added each month, now I will be suing these people.
I would do net 15, with a 10% or more of the overdue balance added each month until paid in full. Service suspended after 30 days late. That will be the first change I make in the 2013 season contracts.
DO NOT let people run up a balance. I learned the hard way.