Hi. Dropping out of lurk to add to this, but I 
just have to add my rant from the other end from 
the bidding/buying one - the receiving end.  I 
don't mind using Priority boxes totally flat, but 
I really hate getting them that way when people 
think that:

1. The glue the post office has already applied 
is the only security needed.  I've gotten boxes 
that opened on one side, and only one little 
brave sliver of glue kept my goodies inside.

2.  If you do tape the ends, then you can forget 
about the glue inside.  No, it will never 
separate, even a little - So, by all means, let 
the cel and drawings float around inside 
freestyle.  I got a box a couple of weeks ago 
that had this, and I'm still delicately peeling 
glue off the edge of a douga and two 
backgrounds...

3.  If you're going to use reinforcing cardboard 
inside, there's no reason why it should be a)as 
big or bigger than the cel/drawings, or b)no 
reason why it should be large enough to extend 
into the "flap" portions of the box.  No, they 
wouldn't bend at all...

Thanks for the "ventage" opportunity!