Easiest way to deal with it is to just ask the seller to send a PayPal invoice to you, listing the goods and postage, for the entire amount that they want you to pay.
First, because you very much want that anyway as it forms the basis of a contract that PayPal can use to refund you, and second because it puts the ball back in their court.
If they want to sell, they'll do it. If you get a huffy response, as I recently did, that they "don't use PayPal invoices", then you've dodged a (probably non functioning) bullet.