Had to take a few deep breaths, many memories of ruined commissions, after hours upon hours of work, destroyed by the seemingly simple act of spraying them with a fixative :O
Ok, firstly, what type of paper are you working on? A well toothed paper will hold the pastel pigment quite solidly and you'll find that they don't smudge too badly. I will occasionally spray a very thin coat of fixative in the bottom layers of my portraits to hold them a little more firmly, but never on the top few layers.
Fixatives tend to dull and tarnish a pastel painting, as Equusketch found out (told you so :P ), it tends to make the pigment bind together and look like layers of pastel have come off the paper. Not a good look, and **** near impossible to fix (I've been there... ended up mixing acrylic paint with a little sand, painting over the tarnished area, waiting for it to dry then going back over with pastel. That was the best I could do to fix it)
As for postage (and I work on velour which makes it about a thousand times more fiddly to post than standard toothed papers), I fix my work to a thick piece of plastic coated cardboard with masking tape around the edges. I then stretch a piece of baking/greaseproof paper tightly over the drawing, and secure it firmly to the backing board. Then another layer of cardboard, sealed very firmly to the backing layer. A layer of bubble wrap, and then sandwich that between two sheets of coreflute or cardboard.
The only time I've had a drawing arrive smudged from the post, was a local drawing when I thought I'd do a 'test' run to a friend and sent it with only 2 sheets of cardboard and it got bent in the post.