Possible, but not recommended - Australia Post really don't like it, and have been known to reclassify them as parcels because of the packaging (card / paper or bubble mailers are best for anything going as a letter).
It can be ok on flat, even, rectangular products (like a book, for example), and taped down to fit the contents basically like a glove (i,.e get it as close to vacuum sealing as possible, lol).
