I agree with most that has been said.
From recent experience, I sent two kites by Aus Post. They both were packed in boxes with pumps etc. One was slightly bigger than the other and weighed approximately the same.
The larger box with insurance sent Sydney to Brisbane cost $50.00.
The smaller one with insurance going to Melbourne cost $35.00.
When I queried the price difference the answer was the larger dimensions of the box, (which would have only been a few centremetres longer.)
The inurance is relatively cheap, $11.70 for $1,000, so approx 1% of the value.
Both were categorised Regular Parcel over 500g.
So the smaller the dimensions or irregular packaged shape (within reason) will save you some money.