Alot of people add the postage cost in to the customs value. Even worse, alot of postal employees try to tell you this is the correct way to do it.
"You may have to pay Customs Duty on goods with a value that exceeds £120. You will have to pay Import VAT on goods with a value that exceeds £18. Note that on all goods from outside the EU, Customs Duty is waived if the amount of duty calculated is £7 or under.
The VAT limit of £18 is in regard to the intrinsic value of the item and therefore does not include any associated freight or insurance costs."