Double invoicing and poor service

I have a TNT/Fedex account. I ship regularly..I am waiting for a shipment to be delivered An invoice for duty has been issued on my account - payable in 30 days time. However Fedex have now issued a 2nd invoice for the same amount requiring Cash to be paid before delivery. So I now have been issued 2 invoices for the exact same amount and my shipment is not being delivered Since Fedex took over TNT the service has decline - so many mistakes made and no consequences for staff making them. The only shining light is Michelle my account manager but she seems unable to get any service either