If you mean the "inc. VAT" button on the products page before you actually clicked the "buy" button, then I think it's only for showing how much it is if you were to pay the VAT. The VAT gets calculated anyways weather you click it or not (for Europe at least). The moment you put the United States as the destination the VAT gets set to 0% automatically.
So you didn't overpay, though that's only if you actually mean KeyboardCo and if I didn't totally misunderstand what you mean.