Steve I've imported sh!t loads of stuff from the states mate and it goes a bit like this..........
(I may be a tad out here n there but this is a good benchmark at least)
Up to £18 value = no charges
Up to £36 value and marked as "GIFT" = no charges
Over the above values VAT charges at current rate regardless of "GIFT" markings and it's charged on the full value including shipping costs !
Over £100 and not marked as "GIFT" duty charges applied first (think it's around 5%) and then VAT charges after duty added to value.
Over £100 and marked as "GIFT" VAT charges only.
There is also a "handling fee" from royal mail which is usually £8
I'm sure someone will step in and disagree with all that but after many, many imports from the US I am speaking from experience.
You have to be careful marking down the price as you @rse isn't covered if the parcel goes missing - that being said if you have paid by gift and not paid for insurance you are not covered anyway so take your chance.
My personal rules are........
1.Always try to get the parcel marked as "GIFT"
2.Always tell the seller to just put the value of the parts in the value box NOT value + post (it's amazing how many put the total cost with post included in the value box and then you get screwed for extra charges over here)
3.If it's worth £60 or less then try and get them to mark it down to $50 so you come under the £36 threshold and get no charges.
4.If it's worth more than £60 get the full value down and pay for insurance.
5.Always try and get them to use USPS Priority International service - you can get a shipping quote online yourself so you know if you are being ripped and I have NEVER had a parcel go missing using this service. I think it comes with basic insurance and an extra $100 insurance is only something like $3 extra or so.
But that's just me
Fill yer boots