Overseas postage

Hi.
Wondering if anyone can help me out here.
Been selling for a few months now, but being a bit dozy as i am, i only just discovered a couple of weeks ago that if i use the “Buy Postage” button, then Amazon charge the postage fees to your account and deduct it ultimately from the postage cost that the buyer pays.
This of course was such a relief to learn, as since i started, I’ve been funding all postage costs from my own pocket on the strength of finally getting it back when everyone’s content etc. Being a new seller with very limited start up capital, this has been tough to say the least, so as you can imagine, i was well relieved to discover that i dudnt need to front the postage any longer. Like i said, im a bit dozy.
However, this has been working a treat except for overseas orders which i just can’t seem to figure out how to buy.
It keeps telling me in red small print that there’s no available options based on my preferences.
But when i checked my preferences, there doesn’t seem to be anything obvious that needs changed.
I’ve tried changing weights and dimensions just to test if its perhaps that but still it just keeps saying the same.
Im now wondering if perhaps overseas postage is not available on the “Buy postage” option.
Or maybe im just not doing it right.
Can anyone help us out here?
Thanks