Maybe ask AP, but I had the impression that is not imposed by Australian government or AP, but the US government and it applies to all mail coming to the USA from anywhere.

The $9 is charged, collected and - more importantly - kept by Australia Post.

On the website, they say "due to security changes imposed by the US..." or something to that effect, which is kind of designed to demake people think it's a charge imposed by the US, but that isn't quite the case.

What it actually means is that the US basically said "we're not letting any mail come into this country unless it's been through certain security measures....and we're not going to subject incoming mail to those security measures."

So, the charge is to cover the costs incurred by AP for them to subject that mail to the security measures imposed by the US.

When the US made that change, many countries imposed a similar charge, Japan actually decided not to process outgoing US mail at all for a while. Australia is the only country that I know of that still imposes an additional charge.