The reason that I use the box instead of the 
Priority Mail envelope is that I think that 
a "flat" telegraphs the message to a postal 
worker that "what's inside is FLAT (as in, a 
DOCUMENT);  it can be bent!  So it's OK to bend 
it!".  Whereas with a box, they can't assume that 
what's inside is flat;  it could be an Etch-a-
Sketch or a glass mirror for all they know.  So 
it keeps them from even thinking of bending it.

(All you folks I've bought cels from who use 
Priority Mail boxes, but ship them FLAT instead 
of fastening the ends and using them as a box -- 
you are tempting fate....).