Having for years bought many model Rairoading items from the states, one gets used to the taxman and Post office taking their share - some of the firms I deal with realise the problems and downsize the value of the item to get round the HMRC take.
Sometimes an item will come through without attracting HMRC, depending on where it arrives in the UK. It's all the luck of the Draw.
It was hoped that the current limit of £18.00 duty free would be raised when some changes were made to Import duties - alas the Labour government didn't see fit to apply it to items delivered by post and only raised the limit to those items you brought into the country yourself, i.e. returning from trips abroad.
As for postal charges in the U.S. Those went up a couple of years back and even the average Amirican whinges at the exhorbitant charges being levied.
These postal charges are now making anybody modelling the American railroad scene in the UK think twice about purchasing from the States - I bought a badly needed item from a dealer over there that cost me $7.00 - it cost the dealer $24.36 to send it.
His apologies were most profusive!
For small items from the States, try asking the seller to describe them as Arts and Handicrafts supplies - these don't attract any duties
Cheers
David