If you google the address, does it show an actual premises in Australia?

Personally I always hand write addresses, which overrides any pre-empted recognition by the Aus Post system. I'm guessing the code might be important, but at a minimum, it should show as lodged - did the PO not scan it when you submitted it at the counter?

Not sure how it could suddenly be a US address if you sent it as a domestic item? This makes no sense.

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@michael_cullis wrote:

Just to complicate things, the address the buyer gaves me was Australian (Coolaroo, Vic.) but it was preceded by a code M00113585. As Australia Post didn't recognize the address with that code in front of it I simply mailed it to the Australian address minus the code. Looking at the address to which an eBay label would have sent the CD I found that te label would have shown an adress in the US. I am wondering if the code refers to some sort of forwarding process.

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The Coolaroo Vic address seems was right - it is showing delivered on the AP website.

The only US state to have MO is Missouri - if the first O is not a zero - the code did not come up there.

I'd just wait - see what happens you have proof of postage & proof of delivery.