How can I pay on an Invoice incorrectly billed?!

Avis billed me for time and distance of which they were not supposed to as this is covered by my insurer. I requested this to be removed and be issued with an invoice without the charge so I can pay it. The accounts clerk requested that I should send her communication from the insurer advising of this which I did. Only to later find that my account has been handed over due to payment not being received. <br> <br> I explained over and over again to all the persons they sent after me for payment, that is that easy send me a new invoice and payment will be made. What I don't understand is the difficulty in issuing a corrected invoice... I will not pay on an incorrect invoice, I refuse!