Fedex / non delivery to addressee

FedEx were supposed to deliver a package to a house guest of mine. Instead of bringing the package to the house, they telephoned to say they were leaving it at the local post office, which is 20 minutes away by car, and usually involves waiting in a queue for 40 minutes. When my house guest finally got to the post office window, she was given the package with no further ado. Several days later, when she had left the country, I received a call from FedEx saying there had been customs duty pay (on 3 Obama Tshirts and some Obama stickers), and they expected me to pay. I explained that the recipient of the package was no longer in Italy and that I certainly wasn't going to pay, not least because if they had delivered to the door, as they were paid to do, this problem wouldn't have arisen. Four weeks later I receive a letter with a bill from FedEx, addressed to me. I have returned it unopened to their Milan office. I would like to lodge a formal complaint about their disservice and prevarication.
Kate Singleton