I've had no less than 3 cels severey damaged in 
the mail, and several others arrive with more 
minor damage.

#1 - Second cel I ever bought, a nice little cel 
of Kotori from X.  Due to being shipped in JUST 
an envelope, the cel was bent up, torn, etc.  The 
seller gave me partial compensation since he was 
at fault.  Amazingly, the sketch wasn't damaged 
at all, and I still have that.

#2 - A lovely WK Aya cel had the package WRAPPED 
AROUND the mailbox flag because the post person 
was too lazy to walk the few yards to the porch, 
and couldn't read the DO NOT BEND.  I got 
insurance claim for that, though the PO kept the 
cel.

#3 - A beautiful Alec cel from AnC arrived with a 
large holed stabbed in the middle of the package 
^^;;  Got insurance on that, and the PO even gave 
me the cel back (it's in my gallery)

So if it was insured, look into fileing a claim.  
If not, and if the paint isn't damaged, put it in 
a cel book and it should flatten out some over 
time... I've had a few cels arrive bent, and as 
long at they aren't really really cteased, they 
will be okay.  And DO open the package after you 
talk to the PO, the damage might not be as bad as 
you thought.