Roger, have a look online at
www.e-go.com.au for shipping quotes. I, and others here, have used it for bigger articles and their pricing is good (it's worth doing a mock quote to the furthest Capitol from you to see the max cost). They tend to use Roadrunners, or similar for there shipping. If the buyer has a depot nearby then the price can be reduced over a home delivery. Alternately they will deliver to a work place through the day.
Just a thought, and it gives you more potential customers, and doesn't involve trying to make it fit the Auspost limitations.
Trev