Yeah, shipping pastels is a tricky thing. I've had pretty good luck though. First I put a skin on the glass. This protects the piece should the glass crack. You can buy this through the Airfloat people. Then I use their shipping boxes. I have a couple of the ones they make that have a metal insert in the lid. See the pictures.These are more expensive, but I think worth the extra. Make sure the boxes are large enough. There should be a good amount of cushion all around the piece. I really don't have quite enough margin for the still life! These boxes are great since you just subtract the foam insert where your piece is going to be and put it in, close it up. You don't need to wrap it in bubble or anything. This makes it easier on the show end as they don't have to fuss with packing materials. They really like this! It's so much less hassle for the shipping agent. Tape the edges closed where they've marked it.
Label the box "fragile" and "this end up" and hope for the best! I spray all pieces that I'm going to ship. I put an Fedex airbill in with the piece for return shipping. I have it shipped back the slowest option, like 3rd day. It could be around 75.00 to get it back. Sending it can be cheaper because you can send it UPS if you allow enough time; probably around 40.00.