Here is how I pack mine and have yet to have a
crushed one (hope I am not jinxing myself now
since I just sent out a few packages!!) . . 

When I send out a cel/sketch, I place them in a
waterproof envelope and pack them in a Priority
box (12" X 9" X 3" one) and reinforce the sides by
re-folding the flaps over the main body.  It is
kinda hard to explain, but it works very well as
it would literally take a lot of effort to bend it
 since it has 4 layers of cardboard on the sides.
 In addition, I would always write "Do Not Bend"
in about 120 font on both sides just in case the
post person looks over the "red marks".

Just my 3 cents on how I package them in case
anyone is interested in comparing notes. *shrugs*

But, like I said before, you can't straight away
point a finger at the seller as the original
poster didn't say it was bent directly due to
faulty packaging or what-not.

I have had my share of incidents with horrible
packaging and miraculously the cel wasn't bent
(then again, I have an awesome post lady who knows
how to handle packages).  

I hope all turns out okay, still shivering from
the poster package tire tracks!  OMG!!

Star Phoenix