Odd, I've only ever been charged on the declared value.
You have been very lucky.
UK Customs assess the amount of Duty to be paid (variable percentage depending on the nature of the goods) based on the declared value of the goods
plus the transport costs to the country of destination. VAT is then paid on the total of declared value of the goods
plus the transport costs to the country of destination
plus the Duty!
And then you get to pay the service charge on top of that!
In my experience, getting hit for the duty and VAT is more to do with the size of the box than the anything else! They seem to think that the bigger and heavier it is, the more valuable it's likely to be - they may have a point!
But always the best solution is delivery to somebody who is in the USA and coming back shortly.
Agreed, if at all possible. If they get stopped in the green channel, it's their problem!
SD