I can answer that one. A 3kg satchel is a flat price Australia wide. And postage is $11.70 plus satchel cost. Which may be more than short distance postage, but is way less than other side of the country. And what is 3kg to around the corner?

Use what works. But understand what you are using.

And it is not within your own state, it is within 50km. Try sending to the other side of your state, even Tasmania, and see what the costs are.