Address Not Recognised

I've had a number of instances recently where a customer has purchased an item and on payment has received a message saying that the seller won't post to their address.

 

On all of these occasions the buyer has had an Australian address so payment should go through without a problem.

 

I have been told that sending the buyer an invoice gets round the problem but this doesn't seem to be working now either.

 

Anyone know of any other solutions? I contacted eBay and they were pretty clueless. I notice it always happens on the last item of a multi-quantity listing.