Not much is available on the internet regarding how to ship these more delicate pieces of artwork and so I thought I'd post some information about how I pack my pastel paintings for shipping. I would like to thank Roberta Baer for sharing her idea of the special box used for transporting and shipping pastel paintings. She is an amazing pastelist who's blog I follow at
http://www.robybaer.blogspot.com/.
First, I build what I call a "floater box" (thanks Roby!). When they say a picture is worth a thousand words, this is exactly what they mean! Click on the image for a more detailed view of the box construction.

Here, I used a combination of cardboard and foam core "spacers" and hot glue to build the box. I would not normally use cardboard but for this larger piece, it was necessary due to lack of larger foam core pieces in my studio.
This type of box allows for a space or gap on the front of the pastel painting so that nothing touches it's delicate surface. In essence, the painting "floats" in the box, having only the backing as a support. Note that the pastel painting is mounted onto a slightly larger foam core board with artists' tape. The painting/mounting board then slides in the gap created by the slits of foamcore that have been hotglued together.
I add an extra strip of foam core at each end of the box that fits inside the front/back cardboard pieces. This helps support the ends so that they don't collapse (note white strip beside box below).
I write instructions on the box on how to proceed to open and slide the painting out of the box. The box is opened right side up and at one end where a pull tab is attached the the backing of the pastel painting.

The "floater box" is then packed in a secondary shipping box for added security. Here, I've re-inforced the edges of the box with foam core to make it more sturdy. In this case, I was lucky that the "floater box" fit perfectly snug in the shipping box. Otherwise, place crumbled-up newsprint to fill the gaps around the "floater box" so that it does not move around inside the larger shipping box.

I make sure to mark the box with FRAGILE on all sides.
I hope this helps fellow pastelist with the dilema of shipping or transporting unframed pastel artwork, or at best, provide a cheaper alternative method.
Below is an example of a box I built using 100% foam board:

Keep on painting!