According to the Customs and Excise site £18 is the point at which duty is charged.

Four separate CDs wouldn't have attracted duty (but would have cost more in postage).

I've just looked at Amazon's Terms and Conditions, and it says "In Marketplace, items are offered by sellers for sale at a fixed price ("Fixed Price Sales"). Unless otherwise agreed by us with the seller in advance in writing, the price of any items sold in Marketplace (including the applicable delivery charge) are displayed inclusive of any customs and excise duty, VAT and other taxes which may be applicable and sellers may not charge or seek to charge the buyer for any customs and excise duty, VAT or other taxes which are additional to the price displayed for the item on the Site."

According to Trading Standards that doesn't exclude Parcelforce from charging a handling fee, nor Customs and Excise charging duty.

John