Since I own Frame Destination, I am use to shipping frames and framing supplies. Uline is the best source for large flat boxes which are not standard. We use a combination of bubble and crumpled paper around the frame. It is critical that no side of the frame is near any side of the box. Corners are the most important. We ship glass without trouble but do not ship glass larger than 24x28. For larger pieces we ship acrylic. For mats and foam, even if just a single sheet we still put them in a box. When you send just a flat piece of cardboard it is too easy for it to be propped up against one box and then have another box put on top of it forcing it to be creased. Best way to ship prints is rolled in a tube. No matter how you pack you must expect some damage. If you pack well your damage rate will be low, but not zero. I don't know how, but we have had the shipping company break the acrylic before. When packing keep in mind that the box will riding on a truck for a few days where it is just being shaken the entire time. If you have some items loose inside the box where they can rub against each other, they will be damaged.