Here you go mate, read this article:

http://www.ebay.com.au/gds/DVD-postage-costs-/10000000013611294/g.html

It will give you everything you need to know.

Personally, I just wrap it in my own thin cardboard or one of those large letter envelopes (or the half-size ones) and slap two stamps on it and send it off in the local letterbox.  Always works.  Wrap it in an A4 sheet of paper for extra protection from bumps.