Lots of postal services can be slow and may loose the package but aus post express is what we use as it guarantees same week delivery if posted on monday throughout australia. Aus post is also the only postal sdervice that allows live articles if the package is clearly labelled.
Packaging the bugs is where the most mistakes are made. If the package is going interstate or through an area with high temperatures or extremely low temperatures, issues can arise. for low temperature just adding a layer of aluminium foil around the postage tub is beneficial but for high temperatures, there's not much that can be done. adding more stuffing, using a foam box or adding an ice pack helps a little but the best thing to do is wait a week to see if the temperature lowers. In terms of stuffing, Shredded newspaper is what we used previously as it was cheap, lightweight and stopped the postage tub from rolling around in the package.
The size of the box should be big enough for at least 20mm of filling on each side of the postage enclosure to the box. The aus post store does sell prepaid boxes and satchels but its more economical to find and or buy boxes and then getting a postal sticker or stamps.