Haolove, If you are a big time seller then you can get a contract with Australia Post.  I know that with one buyer I have bought from they never pay more than $10 to post anything.  I know there are restrictions.

Speak to someone at the post office.

I usually offer free postage because I make my own things.  For others I just charge what it actually costs.  I'm not running a business so for some people you need to decide which is better.  Ebay, own website or a retail shop.