Why is Australia Post Shipping/Addition greyed out when trying to set up a carrier in Shopify?

If you try to add 'Australia Post Shipping' as the rate carrier/provider for a zone and it appears 'greyed out', this means you have already added the app to this zone.

You will only ever need to add 'Australia Post Shipping' as a rate carrier once to each zone.

In this example you can see that when we try to add a new rate and select app ('Australia Post Shipping') as the carrier, the option is grey and unavailable.

This means we've already selected the app previously for this zone. If we view the list of rates in the zone 'Australia', as shown below, we can see 'Australia Post Shipping' already listed.

If you can't see 'Australia Post Shipping' in your list of rates, it might be because you have some default Shopify rates, in which case you'll need to click Carrier and app rates to see if 'Australia Post Shipping' is listed.