Promissory guarantor

I was a member of an LLC and signed up for a credit card merchant service. I did everything online and was listed as the promissory guarantor. Since the business has dissolved and all outstanding debts resolved. A month ago I received notice from the merchant service that $13,500 were disputed and I was personally liable. Mail was not forwarded to me and I was eventually able to get copies of the disputed charges. At that time the merchant service said I could no longer provide a rebuttal for the disputed charges. Am I personally liable, is it too late for me to do anything?

You can still dispute it and try to go to court or small claims if need be. They cannot really impose any absolute deadline if there were reasonable reasons you did not get notified.