Write them a short and simple letter about this. Deny whatever you think you do not owe. If they sue you then that is part of your defense.
If the number is high enough you might wish to offer to settle the charges with an accord and satisfaction check. Then the letter you already sent will be the bona fide dispute that the settlement is premised upon when you later make it.
No legal representation exists by virtue of this answer. Consult your attorney. Licensed to practice law in Indiana and Illinois. Circular 230 Disclosure: any U.S. tax advice contained in this communication is not intended or written to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code, or (ii) promoting, marketing or recommending to another party any matters addressed herein.