I've shipped 4 framed and glazed 12x16 prints at a time, all the way across the country using FedEx. I researched the issue by going to or calling stores that sell packing material and do custom packing and crating. The consensus was that I needed to use a box within a box. I packed all 4 framed prints in a box, stacked with layers of 1/2" styrofoam ($12-15 for a 4x8 sheet at Home Depot)between and around them. The inner box was maybe 3" larger in every direction (ie, 6" total in each dimension) than the cube of stacked prints. Pad generously around top, bottom and sides, with peanuts, airbags, or the cheapest, crumpled newspaper. Then, place this box into another bigger box, again 3"-4" of space all around from the inner box, and pad the space in between the boxes with crumpled newspaper. The outer box absorbs any shipping abuse from being transmitted to the inner box with your prints. As I recall, FedEx shipped it all for about $40-$45. When I checked into a custom made crate, I believe they wanted something like $150. All 4 of these prints arrived safe and sound.
Regards,
Dave