I got a bill from UPS that I don't owe? How to fight back?

Last year I was almost scammed. I put my PSP up for sale (but its now not), and this particular person replied. Any the gist is that I foolishly sent the PSP via UPS (I logged into the scammers ups account and entered all the details) to an address in America that wasn't the scammers address. I naively believed them when they said that they would pay up when it arrived, I'm far wiser now though. Anyway, it turns out the address was of some local government official. Anyway after a couple of months of to-ing and fro-ing (I kept asking UPS to send it back to me, but the scammer used the same account to keep it in America) I managed to get my PSP back. However, even though the bill skyrocketed because of the package going back and forth between America and the UK/Ireland, they waived it...or so I thought until I came home today and found the letter telling me I owed them just over one hundred and ten euros. What I want to know is how can I fight this bill if they keep insisting I pay up?