if the item is new and from the states you might have to pay customs normally this is done at the shop before it beening posted,i would think its the same from Japan also the uk customs can open this to see what it is and then charge you the VAT which is payed by the post services then they get the cash back from you.
there is away around this for small items the shop or person you buy it from puts this is a GIFT ie birthday or xmas then you donot have to pay the vat or if you have done,u can get a refund.
i got a GSP for my motorbike from the states it was a great deal but then got stung by the VAT