Well, I'd like someone to tell Mika that some of his fans are not happy about this and that the merchandise supply at his concerts doesn't match demand. It's odd that it is totally US-based, though (presumably because his music publisher is American), as they must ship more stuff to Europe than they sell at home. All sent by air mail ... Is it about maximising profit?
The rules on duty seem to be applied in varying ways, judging by the complaints littering the internet about this, especially when people send presents to others that then get hit with tax (and the fee). Perhaps posters are VAT free - but you may have just been lucky!
The sticker on the parcel suggested visiting www.hmrc.gov.uk and searching for "Notice 43".