I'm a bit late to the party with this question. I have recently started selling greeting cards on eBay and the “Australia Post Standard Domestic Letter untracked” comes up as an option. How did you change it to Australia Post Standard Domestic Letter Tracked?  Or did you just do that directly through the post office (not via eBay) and provide the customer with tracking number?

Australia Post tracked envlopes for sale | eBay

If you are using flat postage, as you should be if you're sending as letters, there is an option next to the postage method to change service. Do so.

If it doesn't work first time, reload the listing page.

That was scary 😧 

I really hope it’s not something they are trying to phase out x

It must have been. You took FOUR months to get over it and post a meaningless reply.