Not really secrets but might help someone: 

I noticed that no one has mentioned www.baggagefreight.com.au so figured I would flag it in case people are not aware.

You can almost always get a better price than AP for bigger items 

Alaso depends what you sell but a vacuum packer may be a worth considering - its surprising what fits through the large letter slot when all the air is sucked out - it is possibe then to save $5 on a $7 postage item.