I can attest to this.. I shipped some acrylic displays to a lady via Fed-ex ground. There was "Fragile tape" on every seem and a "This end up" sticker. When they arrived the box was crushed and the displays were totally messed up..

I later found out from the maker of the plastic that in order to break the Acrylic I had used is would take 1200lbs pre-square inch of continuous load to break them or a sudden impact of 900 lbs. The box had a total crush depth of 6".. and they had the balls to deny my claim stating there was no damage to the box..