If you are running your own Shopify website, are based in Australia, and are looking to ship using Australia post (after trying almost every Shopify addon) we can highly recommend Australia Post Rates & Labels by PluginHive.
This addon allows you to sync with your Australia Post business account, and calculate on the fly pricing in your admin area and more importantly, generate Australia Post labels to print.
Setting up an Australia Post account is a little tricky, but once you do, you can then create your developer account to create your API Credentials that are also required to sync between Shopify and Australia Post.
Australia Post Rates & Labels allows you to easily create an Australia Post shipping Label to print and can also generate an Australia Post Manifest if you wanted.
It also does all this inside your Shopify account, so there is no need to click through to a third party website to manage your shipping.
Once you generate a label, as they are output in PDF format, you can simply download them, open them in Acrobat and scale to fit your printer without having to manually resize them.
If you generate lot of labels, we can also recommend using a Dymo 4XL to print them, and if you churn through a lot, get yourself some compatible Dymo Compatible 4XL Extra Large Shipping Labels which you can find easily enough on eBay. For Express postage, you will also need Express postage stickers / labels which you can get from an Australia Post Business Hub or on eBay.
What we can also say is when we had issues with this addon, their support team was outstanding, either answering our questions, or patching / fixing issues we can across in a very short amount of time, and also sending follow up emails to ensure everything was running smoothly.
The only minor annoyance we have with this addon is that there is a security issue (cross origin) when trying to download labels using Firefox, and you would need to disable the security.csp.enable in the about: config menu to allow it to work, so unless you want to do that, its easier just to Chrome instead.