Such cardboard can be bent especialy if the packet 
get stuck between two others. If it does get bent, 
you can be sure that the cel will be destroyed.

No, there is something that's pretty good but I 
dunno if it can be found outside of Japan : it's 
very thin plastic  boxes that are pretty light.
This really can't be bent and there is the right 
size for regular cel without background or with 
background. (even the one with background will fit 
 in a "Fedpac". The problem is to get them :(

Cheers,
Yann Stettler