Fedex unable to explain charges yet insist on payment!

Pathetic service , I am being charged R370 for my parcel that is valued and states on the parcel R320. They are charging me more than 100% for customs duty and customs clearance when this has already been paid for by the seller and already cleared in Johannesburgh. It is now sitting in Durban where it is being held back and I am being harassed for payment. None of the consultants can explain what exactly I am being charged so much for when asked , but 2 out of the 3 consultants agree there is a problem and that rectification has been requested from their side via emails. The communication is so bad between fedex departments that even though I have been told it has been sent for rectification and there is a problem with their costing, the departments sending me requests for payment clearly don't know any of this as they continue to send me emails for payment that they clearly cannot explain. I am not about to pay R370 for a R320 parcel just because fedex said so but are unable to provide me with a clear reason why! Absolutely frustrating to think that this is supposed to be professional business !