One option you're not using is the 1kg flat rate box. You can fit up to 1 kg in the box for $7.15 + around $2 for the box - usually costs under $10 all up and it is tracked. Just print label from ebay and buy box from Auspost.

I notice you sell DVDs. If used DVDs, put small piece of bubblewrap inside the case so it is snug, put plastic sleeve on it, and wrap in brown paper. Put $2 of stamps on. Never had a problem. CDs can be sent - wrap in bubble wrap, brown paper and $2 stamps. I do the same for books on or around 20mm - post from $2 to $4.

NEVER place these over the counter at Auspost - put them straight into red box. If they're slightly marginal you don't want counter Nazis carrying on.

You can buy excellent brown paper that is waxed from Kmart for $5 and I use their 100 pack of plastic sleeves for folders that is $4.