FedEx - Charged the false amount in invoice

I was running an account with FedEx from more than a year. I was shipping products in Cod Service for which they have to charge the amount of Cod service and freight service. They Charged me higher and gave wrong Weight as mentioned my be in shipment and sended me an invoice. When i was going to make a payment then i saw that the invoice value was higher than was supposed to be. So i called at FedEx they gave me the number of account representative and told to ask them for this. I called there but no one responded. Again and again i called them but there was no response from that side. After ** days i got another number of representative then he picked my call and told me to mail him the situation. I mailed him but till yet i didn't got any resolution. And even they blocked my FedEx account on **-November-****. So who will be responsible for my sales which has been stopped due to their delays.