I was on the seller side of the exact same 
situation with one of my cels. the girl who 
bought it from me was extremely distraught 
because it was a dream cel of hers and I felt 
terrible about it, but she didn't want to let go 
of the cel and didn't want me to have to 
reimburse her for it because it was the USPS' 
fault. I gave her the URL to a place that 
restores cels, and she is looking into this - she 
is set on recovering it. I really hope they are 
able to help her! ;_;

/Mina