Set up an Auspost business account - it's free. Then download the app.
Read the Auspost business webpages, there are discounts (called bands) which are based on how much parcel postage you buy over a rolling 8 week average.
Can I print my own shipping labels? If so, is it cheaper, more expensive or the same? Yes - if you buy postage at the post office you'll need to show your code on the Auspost app to get the discount.
Is using Aus Post boxes ever a cheaper way of doing it? There’s a couple of sets that will fit nicely in the medium box, but box plus shipping appears to be in the $15 range. I buy my auspost boxes off facebook marketplace from people bailing out of their "side hustle". The benefit of the boxes are the flat postage rate for up to 5kg based on the box size designation. and you get discounted postage.
If I sell on eBay, is purchasing shipping through them cheaper than going through Aus Post? if you use the ebay shipping label - ebay gets the auspost discount....
Is there a carrier that beats Aus Post? Like, maybe in certain situations? You can try Sendle.... I use Auspost because we all know it can be shite and customers are generally forgiving of auspost delivery delays
Is there a good supplier of boxes for shipping that I should know of? Newpack is quite good always compare their eBay store with the website as the prices can vary. And FBM searching for Australia Post, mailing box, postage post. As a meaningless flex - last week I picked up 40 Auspost large boxes RRP ~$80 for $30.