As mentioned, you're the one who needs to do their homework on this. Everything comes down to what you are selling. Sometimes it's cost effective to use Aus Post packaging, sometimes it's better to use your own.

For me personally, I don't use the pre-paid satchels as they are a fixed price and you only get a small discount for buying in bulk. Instead, I send 90% of my items in my own plain white satchels which go by weight. If you set up an Aus Post Business account and send a fair bit, you can really save heaps off the postage sticker price - up to 40% domestically, depending on the band level you are on and where it is going to.

I use my own satchels as I buy them 1000 at a time and end up paying something like 5-7c each, compared to the other Aus Post satchels which you have to pay 25c + shipping.

These options may not suit your business model and items though.

If you are sending low weight flat items, then look at the A4 green domestic letter as you get tracking and it costs about $6.00. However, if someone purchases multiple items, then you'll need to change from a domestic letter to a parcel due to thickness.

When I first started several years ago, I took some of my items to the PO so that I could work out the best shipping options both domestically and internationally, as well as how the prices and packaging options changed depending on weight and thickness and therefore how to charge the buyer accordingly.

There are lots and lots of options and sometimes the best way to learn is by trial and error.