On 2004 I signed a rental agreement for a unit at Public Storage. The contract gave me a graceperiod of ten days. Eventually, they changed the grace period to seven days. They started to charge me eighteen dollars on the seventh of each month. I called and told them that I had a written contract that gave me ten days and that they could not change grace period stated in my original written contract. Last month they charged me a late fee on the sixth of the month. Can they change the terms of my contract which gave me ten days of grace period? I thought that any changes to a written agreement had to be made in writing. What can I do?