You didnt pay for item and therefore you shouldnt pay any extra charges. If FedEx cannot contact the company who sent the package and find out who originally paid for delivery and contact them then it is their fault. You have nothing to fear and nothing to pay for because the whole incident doesnt involve you.
Sorry but that is totally incorrect, FedEx are just collecting import duty on behalf of customs and excise. True the initial fault is with regards to the person who sent it and they probably didn't even think at the time this would be liable to duty.
By accepting the goods then the op has made herself liable to the charge even though she did not know there was a charge.
I am surprised that FedEx do this retrospectively as if they had notified at the time that there was an amount payable then the op would have had the chance rejecting it. Certainly for commercial shipments FedEx alert you at the point of arrival and ask you for customs clearance instructions. I don't know if they would deliver and then bill afterwards as any charges we pay at work we put on our customs deferment account at the time.