UK Shipping, I didn't Know...

Sometimes you can miss paying import tax they don't pick it up at the sorting office & that's fine. But if they do, you also have to pay a handling fee!! I think it's 20% of the value if the item including shipping, plus 17% vat if it's over a certain monetary threshold? As said above, if you list the value of the item as lower, import tax should be less. I've been stung a few times buying from the USA,