What is the item, maybe it's restricted? It's not the Armadillo left arm part by chance? This description could be causing the issue perhaps?
Perhaps the buyer has 2 or more unpaid strikes and has hit one of your blocks?
Have you tried sending an invoice with the correct international postage?
Go right of the item sold to More Actions, and select Send payment Details, amend the International postage there to that required, but do not click on reset. Check it is correct, and send .
If you are on the Seller Hub, you click on the item in Orders >> Awaiting Payment, then top left you will see send Invoice, follow the above instructions.
If this doesn't help you may need to cancel the order, go right of the item sold and from More Actions select Cancel this Order selecting the reason Problem with Buyer's Address.
@spudulike2