If you fulfilled the contractual requirement of sending your termination letter by certified mail, return receipt requested, send them a copy of the return receipt and remind them that you cancelled as of the next dies payment date. If you contracted to pay late fees in the amount of "five percent (5%) of the payment or five dollars ($5.00) whichever is less", then that's what you agreed to pay. I don't think a court would deem a late fee of less than $5 as unreasonable; you're free to try to litigate the issue.