Buyer should edit the delivery address BEFORE paying.
However if you do not post to USA I think he will have difficulty buying.
Would it not make a lot more sense if he arranged for friend in the UK to buy direct?
If paid from a US account, then you do not have to worry about Dollars, but selling outside the UK there is a charge.
International fee
If your registered address is in the UK, we charge an international fee of 3% if the delivery address for the item (entered by the buyer during checkout) is outside the UK.
This fee is calculated on the total amount of the sale and is automatically deducted from your sales proceeds. The total amount of the sale includes the item price, any handling charges, postage, and any applicable taxes.