Posting by weight but a light item is really expensive

[shoulders]

hello

basically most of my stuff is heavy and can have its postage calculated by weight but i have some items that do no weigh much and are worth a lot of money.

So how do i keep calculating shipping by weight but get these expensive items to be shipped with appropiate shipping costs (ie shipping + £100 insurance). I also have items in the same weight bracket that are not expensive and would not need the extra insurance.

what i need really is to calculate shipping by weight, and then if the item is not heavy and within a certain price range, a shipping option with insurance is forced.

thanks