If you are shipping framed artwork to an exhibit or gallery requesting re-usable packaging, I highly recommend the Strongbox by Airfloat Systems, Inc. These sturdy cardboard boxes come with three layers of foam. The top and bottom layers are egg crate design, with the middle layer made up of perforated 1 1/4" squares. Punch out the size of your framed work and drop it in. The work is not only sandwiched but protected on all four sides by shock absorbing foam. The sizes start at 17"x22 1/2"x3" and go up to 62"x62"x5". You can choose lined or unlined - lined includes lightweight plastic puncture guards with the strength of 3/8" plywood.
The Strongbox is not inexpensive, they start at $38.00, but they can be used over and over again. Every year I travel, by plane, to a seminar where I bring at least one piece of art for display. I've been using the same Strongbox for five years. I put a handle on it and check it as baggage (still free on Southwest). Price aside these boxes work - they protect your art, they are lightweight, look professional and exhibit coordinators will love you.