Right, i have just gone through this too.
On importing wheels from the usa, you have to pay the price of the wheels plus postage, then add 4% on then add the 17.5 %. i only know this as i rung up customs and excise the other day to verify.
I also found that renting a us address was the cheapest option as you pay the least postage which decreases your customs charges if you get caught, which is most likely if you go through a well known courier as everything gets checked by them(from what dhl, parcels to go, ups and bax global told me)
also, bax global are the cheapest importer, however, they only import brand new goods from a us repitable business address and wont import anything thats a gift or personal.
I would say as they are such huge parcels i think customs will 99.9% see them but thats just what i think.