FedEx Charges Unexplained

I am importing a shipment from the USA and have received an invoice from the Service Provider. It's been two weeks I am asking for clarity as to why Vat on R1555,90 duty ends up being R803,70 (which works out to 51%), they are insisting that the calculation is correct and I have been threatened that if I do not pay within the next 5 days, then my shipment is going to be destroyed. The explanation I got is that Customs also charges an additional 10%, which would mean I have to pay 25% instead of 15% on the Vat. But even this explanation does not yield the R803,70 on the invoice.