I did keep my calm.  I even read through my messages before hitting send.  And I did refund them straight away.

But I just wanted them to see that I had in fact paid for postage and had sorted it out with Aust Post and that they agreed that it was them that was at fault and not myself.

And even with an immediate apology and an immediate refund of they payment they made to Aust Post I was still called a liar, a cheat and a thief