The Proprietary Lease must be amended to include a late fee clause with the amount that would be charged, i.e., a percent amount or a fixed amout such as $50.

However, your co-op needs the money to pay bills and provide you with basic services as well as pay the salary of those who take care of the buildingd. If the rent is due on the 1st of each month and you have 10 days grace period to pay your rent, I would suggest to do your best effort to pay on time. If for some reason you are experiencing a problem, approach the Board with a letter explaining the issues you may have so that a greater period of time is considered in your case. If this is the first time you are late, then request a waiver. It usually works!