Hey Debbeye, Avery-
I bought a cel from you, and I love it very 
much.  To be honest though, my box was somewhat 
banged up and bent, even though you did mark it 
do not bend, and it was insured.  As far as I 
could tell the only damage was some sticking of 
one layer that wasn't stuck before (at least not 
from the auction description) and the package was 
insured- the postal worker noted the damage to 
the box in case the cel was ruined.
As a seller, I think if you insure for the full 
value of the cel, you are doing your job.  
However, an extra layer of cardboard could be 
helpful too.
What I have done in the past is a priority mail 
envelope, middle size flat priority box inside a 
large flat priority box, sealed with packing 
tape.  Lately I started doing just the first two, 
but if people want it I'll do the third box. 
One thing I would add- NEVER SHIP CELS WITHOUT A 
CEL BAG.  :)