I think you mean the plastic box that Celga or 
starbreaker used. Though it can't be bent, it is 
fragile. The corner and the edge are two places 
that earily broken into pieces. If you are 
sending to US which the post office staff are 
rude to parcel, I won't recommend you to use it.
But I've to say they are cheap, you can get one 
in a 100Yen shop in Japan. Also, it really 
depends on the plastic used. I have got one which 
are really tough and I think it will not break 
even if you throw it to the ground(but I suppose 
no one will to do it). And more important, it is 
big enough to host most of the over-sized cels. ^^