Excise duty and import VAT- these are the things you should look out when purchasing items abroad. I have been charge with this when I bought an item online from U.S.A, unaware that goods with a value of £18 or more would be charge with customs charges, plus the handling charge of the postal service which is around £8. But if you only purchase and item within the EU, imports duties do not usually apply. Most websites would post their regulations about duties and import VAT and is usually showed before you check out, but some online seller would sometime post this on a very tiny font somewhere on their web page, and most buyers would not bother to read it. So, it would be a surprise when the letter that contains charges is received.
Now another thing, when receiving a gift from abroad, you can be charge with import duties too, but that is if the amount is £40 or more. I do not know how they would calculate duties charges but I think they would base it on the amount of the goods sent plus the shipping cost.