The older ones are of a fairly rigid plastic which can cause them to snap if handled too roughly. Mediashelving, for example, doesn't use any packing material when they ship out a box of 100 and if FedEx/UPS jostles the box around too much the ones on the edge can get damaged. As such, shipping them via USPS in a regular envelope may cause this to happen as well depending on how they treat the parcel.
The newer cases that just came out, however, seem to be of a bit more malleable plastic so they may withstand a bit more bending before they break. That's just me speculating, though.