There are flat rate satchels (and boxes), which can be purchased online from Australia Post (they have a store here on eBay: http://stores.ebay.com.au/Australia-Post-Store which has a bit of info about their products), the satchels are the same size a pre-paid satchels from Australia Post, and you can pay the postage for them as you go by using the Click & Send website, or you can process them on eBay have the cost charged to your  account and pay when due. 

On Click & Send (the website), and here on eBay, you can also process and print labels for parcels in your own packaging, so which one works out to be better for you may depend on convenience or cost. 

Both need to have a label printed out and attached to the parcel, but you can't (or shouldn't) put an eBay flat rate label on a parcel that's in your own packaging. (You need to enter a code to use flat rate options, or select own packaging).

There are some differences between using flat rate satchels and own packaging, particularly if you use the C&S site, not eBay, to process labels, such as payment methods, but I hope that clarfies things a little.