It has recently been pointed out to me by a few followers on Twitter that on the 1st November 2011 HM Revenue & Customs lowered the import value from £18 (around $25.50) to £15 (around $23).
Basically what this means is that if you were to purchase something from America (or from outside the EU) that is over £15 then you are subject to Customs Tax
So if you are going to buy something from Amazon USA that is over £15 and you do not pay the Custom Tax despoit then you will likely be charged by Customs when the item arrives through the mail. Some website such as Planet Axel post the item directly into Royal Mail (or your countries postal method) so you will not be charged.
Additionally if the item was purchased privately, such as on eBay, you can request the seller to send it as a GIFT, which then allows a Customs value of up to £40 - yet again if the item is marked higher than £40 on the box then you will be subject to customs charge.