For example, jewellery - looking to buy something in the States, but it mentions on the site that it could be liable to duty of a not insignificant amount. Would the package be inspected somewhere in the delivery process and a tax bill slapped on it? Or should I declare it? I'm planning to have it shipped to a friend in the US who is coming over and can bring it with them - presumably they can stick it in their suitcase and not worry about it right? Or should they declare it?

Literally have no idea.