The piece of bubble wrap inside the cover is a good idea - anything that might prevent the dvd moving around or getting scratched.

If I didn't want to invest upfront in a bulk quantity of cardboard mailers I would probably invest in a quantity of the prepaid envelopes from Australia Post and then use some cardboard either side of the cover - if I had a few mm to spare. Chop up a few boxes, it is cheap, and you get to do a bit of 'recycling'.