I believe the shipper paid for all charges relating to this shipment, so why have I been charged? 

The shipper states who will pay the duties, taxes and other charges using the appropriate Incoterms® – they state this on the air waybill and commercial invoice. FedEx follows this instruction and may have charged you if it was indicated that the receiver pays the duties, taxes and other charges.

If you believe that the shipper should pay these charges, you will need to discuss this with them. The shipper must ensure their instructions are correct and clearly indicated on the paperwork for future shipments.