I'd been meaning to create this post for a while now. I've had these pics since last summer!
Now and then I get asked how I mail unframed pastels and keep them safe. I've actually been sending unframed pastel commissioned portraits for years, most of which are on paper, and matted, usually 16x20. In the case of those, I put a piece of glassine or wax paper between the portrait and the mat, slide that into a Clear Bag, and wrap the whole thing up with plenty of thick cardboard until I have a flat 'box' about 17x21x1. I've never had any arrive damaged.
For the small pastel paintings that I post (almost) daily and auction off, I have a approach that I modified from Carol Marine's method of shipping her small oil paintings. I just add glassine and skip the little tabs that hold the lid off the painting.
NOTE: I've since gotten into the habit of ordering the 9.5" x 12.5" sturdy Priority Mail Flat Rate envelopes, and use those for all 5x7's and 6x6's, and if I make the sandwich edges narrow enough, even the 6x8's will fit. I also purchased a roll of the slightly-sticky bubble wrap, the kind that will wrap around an object and hold on! I love it!
If you're shipping a larger unframed pastel painting, I would follow all these same steps, but I'd use the large bubble wrap instead of the small and then wrap in cardboard. OR make an investment in some Airfloat Printpads. They're designed to protect and would probably last through a few uses.
Let me know if this is helpful!