OK as a seller the postage in Australia is a mine field, as you can never get the same costings for posting a parcel within Australia example I had two exact parcels same size same weight going to the same post code just different addresses the 1st parcel cost $6.20  went in an hour later with the second parcel $10.20 a difference of $4 , so as a seller more often than not you are having to over quote your postage by some margin due to Australia post Staff that ultimately are the final postage price barrier, so you are damned if you do and if you don't.

So now I use the pre-priced ebay 500g and 3kg bags as they're a set price and if I am sending something that doesn't fit the 3kg bag it goes into a B4 box which is a set price too and make sure that the postage is show clearly on the ebay listing and offer cheaper postage for other item they may buy,

I have found that to be more effective than pulling my hair out and spending time arguing over a mere few dollars.

So I guess what I am trying say is not rocket science  in business you can only satisfy some of the people most of the time and none of all the people all the time!.

If you get 1 or 2 negative feedback in 500 transactions you will be well on your way to doing the right thing by your customers.

Also most buyers today are more savvy now and are not going to buy from you if you are charging more than you should be they know and are quick to let you know it too.