We currently use Aust Post. Haven't diversified into couriers as many buyers don't like them. If you are not home there can be issues trying to pick up the item at a later date and they are not as easy to get to as a Post Office, which for most people are pretty accessible.

AP go pretty much all over the Australia, don't think there is any other service that covers the country like they do. 

And I think $7.15 for 500g etc, is the lowest we have seen for post, and buy your bags in bulk, just increase the quantity that you buy over time as you get bigger and that will save you more and more. The only problem with bulk purchases is that you have to have the room to store it all.

This is just what we do, some other sellers might have some other suggestions.