Hi SMC! 

I post a lot of 'flat' items too, so I use a lot of envelopes with stamps.  The rigid envelopes you are talking about are great protection for posting things like phone / ipad cases etc but they are not pre-paid - you will need to put stamps on them.  You CAN buy prepaid ones from Australia Post, but in my opinion it's not good value for money.

This is Aus Post's guide on what they call "large letters" - it will tell you how many stamps to put on what.

Hope it helps 😄