I prepaid duty and tax charges before delivery, so why have you charged me?

Tax is generally applied by the authorities at the time of import. If you have paid the tax to your supplier and this is not clearly indicated on the paperwork supplied by the shipper, then FedEx will not be aware of this.

You may need to discuss this with your supplier or seller for future shipments to ensure the correct billing terms are selected when completing the FedEx air waybill, and the appropriate Incoterms® are used on the commercial invoice.