eBay flat rate satchel prices are about 10% cheaper than the normal flat rate you buy over the counter, but you do need to buy the satchels and print out the labels yourself - the cost of the postage is added to your eBay fees - the BX flat rate boxes can also be very good value for items that fit, as the same thing over the counter (or through Click & Send) is quite a bit higher.

They have a store on eBay if you want to check out AP's flat rate range, http://stores.ebay.com.au/Australia-Post-Store