As others have said, if someone doesn't pay up, open an unpaid item case after 4 days, don't be strung along for weeks.

As for limiting postage. My sister & i also thought we were covered a while back. She listed items as posts to Australia only.

We had an international buyer & yes, nothing but trouble. We too were surprised as we thought we had limited our sales to Aust but apparently not.

It seems that posts to Australia only doesn't block international buyers, so my sister went back into the settings, she found the right place and had to tweak it more specifically to exclude areas.

If you haven't done that yet, you'll need to as well.