CCO Mortgage Review

How about this story - I don't see what my mistake is.  We have had a mortgage with CCO since 2003.  Had our payments and extra deducted every month.  No problem until the very end - their collections office has called us 3 times and sent us one letter claiming that we have a late payment and they will report us/have reported us to the credit bureaus. Why?  Because our final payoff amount was last month and was less than our regular payment amount.  They did not deduct it from our Charter One checking account as they have since 2003.  They did not send us a letter telling us to make the final payoff separately.  Just phone calls and a letter accusing us of a late payment and threatening reporting us to the credit bureaus.  Eventually I spoke to customer service who figured out the problem, asked me to send a check in by mail for the final payoff.  I went to the post office within half an hour to send the check by certified mail. Then the next day - another phone call from the collections office.  WTF.  I can't call them again until Tuesday due to the long weekend but I will be furious if they reported us as delinquent to the credit bureaus when it's their fault for not taking the money out of our account.  No problems for 6 years, and at the end their ineptitude could damage our (pristine) credit score.