If goods are sent through courier services such as UPS, they are contracted on behalf of the UK Customs and Excise to collect the duty and VAT, and they probably add an admin fee to this. The courier will collect the payment from you when the item is delivered to your door. As a private company that does not want to lose this nice little agreement with HMC&E, they are pretty thorough about always collecting the duty.
If the item is sent through the mail, it will be delivered by the Royal Mail. thety aren't a private company so are less concerned about being so thorough. 3 times out of 4 they don't bother to collect the duty from you.
The duty is calculated based on the declared value on the outside. I suppose if they were suspicious that it was under-declared thay might open it up to check.
There we are: my first post here and only a little longwinded
