Sorry missed your location as Dublin, but same deal, you pay to your country’s customs people, and it’s Ireland that sets the duty levels.
D&T payment is based on whatever was stated on the airway bill. If shipper ticks ‘recipient’, the FedEx follows that instruction and bills you. Alternatively, Shipper could also have selected ‘shipper’ on the awb (they get billed themselves), or even ‘3rd party’ and someone else named by the shipper gets billed.
If your shipment came from Japan, it’s likely to have transited several other countries -possibly DE, FR, UK on its way to Dublin depending on flight routing, but only transited , ie not customs cleared in anywhere else than Ireland.
As mentioned , if it’s not what you agreed with the shipper, reject payment and FedEx will revert to shipper for payment. Be aware that it then becomes an issue between you and the shipper and FedEx is out if that particular negotiation. Lastly, FedEx will get the money back it’s has already paid to IE customs - from someone.