With smaller prints, it's easier and more economical to send them in flat, strong cardboard envelopes. We use white rigid cardboard mailers. They're very very stiff, relatively inexpensive and are available in A5, A4 and A3 sizes.
We suggest not using the provided sticky tab on the back of these envelopes to close them, but instead simply closing the flap and taping down securely. The reason for this is that the glue used on the sticky tab is really hard to open and can cause damage to prints upon opening.
Within these we use our ClearBags for extra protection and pretty presentation, and we place the print plus a backing board into each bag.
Once you get over A3 size, it's easier and safer to roll your prints and place into cardboard tubes.
The main things to consider are:
Using these methods we send thousands of prints each year and in a decade of doing this I can think of only one or two instances of damage with prints shipped within Australia, which were both due to truly absurd mishandling by the shipping company.
Your website is the best. The most extensive and helpful information I've ever come across.