The whole point of international delivery is that there's agreements in place to avoid that, something coming from the US shouldn't be costing you extra for delivery in the UK as that's already accounted for.

Where it's an issue is that RM add £8 for filing for import tax for you so items at the £15 threshold for having to pay tax wind up costing you £11 in extra fees.

Where things get more complicated though is they work out the cost of the item, which isn't always reasonable(I got 2 t-shirts for ~£9 each and a free t-shirt because of an offer, they charged me import duty as if I'd paid £9 each for 3 shirts and it simply wasn't worth fighting it for the extra £2 they stole).

I can easily see this being something similar.