https://auspost.com.au/sending/send-within-australia/compare-letter-services/letter-tracking

I use these a lot for postcards or paper ephemera with a value of over say $20. They are not stiff (obviously) so you would need to add your cardboard and keep under 5mm thickness (which I assume or do already). Because I am lazy I use one of those stiff

Card Mailer #0D DL 220x110mm 300gsm (eBay) which fits neatly inside the trackable envelope and when bought in bulk add only about 13c to your mailing cost. I charge $3.30 for a trackable envelope.

If you need something thicker I send badges etc in a superflat mailing box bought on eBay (many size options which can go at large letter rate ) - which posts as a large letter for $2.20 for under 125g.  You can add registered post for $4.30 - reduces to $3.60 if you buy a box of 50 labels. I charge $6.50.