I can't believe that anyone who sells on eBay would be worried about Australia Post profits. Take it from me, they care nothing about you, except how to rip you off even more than they do already.

Look at it this way, if they can make a profit on what they charge for a 3kg satchel (and they do, don't worry about that), why does it cost the same for a parcel that weighs a third of that and why do you have to go all the way down to a sixth of that weight (500gms) before the cost reduces by a half? Why is it not just over a sixth of the cost (allowing for the cost of satchel manufacture)? The answer is that they use that extra profit to subsidise high volume business customers. If it costs me and you and most every other incidental user a certain amount to send a parcel of a certain weight, then it should be the same amount for a corporate user no matter how many parcels they send. Anything else is at the same level as "ticket scalping".

As for the person, who sold you the items in question, they set a price for shipping (not postage) and then discounted it by $2.00 and you are complaining about them because they managed to get one over on Australia Post. If they had sent the items separately, they would have been forced to charge you the full amount which likely would have been greater than $11.00. Seems to me, you got a portion of the difference in postage cost and if you consider that you still paid to much for postage then you should be complaining to AP about their unfair pricing structure, not this seller.