A super-quick update on this as I'm just about to head out the door...

The buyer has been in touch and we're trying to arrange the return (phew! she was out of the office when it was delivered, but the package is unopened so hopefully she can RTS with no problems).

Australia Post has responded (via Facebook and the case) and offered a refund of the difference between regular airmail and Express courier - I'm not 100% satisfied with that, for various reasons, but at this point I'm in "minimise the loss with the least amount of hassle as possible" mode, so I'll accept it.

However, they did have the nerve to say it was a goodwill gesture because I receive a discount on my postage (via the Business 250 membership). That ticked me off, because a discount has nothing to do with anything - unless they are trying to say that receiving a discount (that they offer and provide) means I can't expect the same service and entitlements as someone who pays full price. I bet I couldn't get away with that with a discount - i.e. send only half the goods I advertised because I gave a 50% discount. 

Dear Aus Post, if anyone from there happens to be reading this, it's not actually a discount if you're not offering or providing the same thing at the lower rate.