AMS
You need to check UK vs USA prices carefully. It can be worth doing it, but not always!
I buy by mail order from the USA quite often. Sometimes, if it's only a little packet, it arrives with nothing to pay. That's rare.
Normally it's a token percentage or zero import duty, then 17.5% VAT on the whole lot (including shipping).
It's very much preferable to have stuff sent by "normal" post rather than DHL, Fedex etc. The "carriers" will charge maybe $15 shipping and handling, then declare a higher number to Customs, and then charge you yet another fee for "handling" Customs. The PO charges a token (relatively small) amount. The difference can be significant - not a problem if you're in a hurry, but annoying if you're not.