I was just wondering whether you have claimed this customs charge back from the company you purchased from?
I had a similar experience recently. I ordered a few products online and was hit with a €14 Customs charge (including the €6 An Post charge). I had to pay the tax before the package was released. As I could not physically get to the depot where my parcel was being stored, I sent a postal order for €14 to the depot and they forwarded the parcel to my local sorting office. The very helpful member of An Post I was dealing with suggested I should write to the company looking for a refund of the customs duty (this hadn't even crossed my mind). At first I was a little disinclined to do this as I figured I would never get any money back but then I decided why not! So, I sent them a letter, enclosing the original An Post slip advising me that customs duty was due and the sticker from the parcel which showed I had paid the duty. That was two weeks ago and yesterday I noticed when in my online banking that the company have refunded the €14 onto my CC. I'm very happy with the whole thing. Of course, getting the €14 back doesn't compensate for the price of the postal order or the hassle of getting to the sorting office but I still think it shows pretty good customer service.
Noor