EBay doesn't like it , but you could just send her a Paypal invoice, off eBay, with the correct shipping charge. Be sure to indicate that this is an eBay item and give the transaction number on the invoice. Then mark it paid and shipped on your eBay pages, since it won't transfer automatically.
If you get an invoice number when you print your PP shipping label (if you do) give that number to the customer as reassurance. The invoice number is not otherwise useful for lettermail, light packet and small packet services.
Then go through your listing, by hitting Sell Similar and look for the problem. You don't have to complete the listing, this is just a learning exercise.
Is the problem that you do not have a specific price for that specific country? For example, although I sell worldwide, I get requests for shipping costs from Mexico, which I do not check off the Custom Destinations, and from countries like New Zealand, which have no checkoff available.