Sometimes international customers can run into issues with some of our listings. The most common way to resolve this issue is to remove a lot of the "Automated Process" by having them follow the below process along with you.
1. Have the customer go to our listing and use the "Make an Offer" and send an offer $1 below our list price (or whichever price may have been previously discussed)
2. When you receive the alert that the offer was made, go to the offers page and accept their offer.
3. Go to our "Awaiting Payment" Page and then send the customer an invoice with shipping cost added.
4. Send the customer a message letting them know we have accepted the offer and sent over their invoice with shipping added. They should now be able to process the payment for their order.
References:
How to Send an Invoice on eBay
How to Calculate a Parcel International Shipping Quote