Some people prefer to leave the cel in its cel 
bag so that the archival tape won't touch the cel 
itself.  However, they should have known how to 
remove the tissue paper without damaging the cel 
(i.e. simply using water).

I really hope you're using UV-resistant glass and 
acid-free (archival) matting.  If the glass is 
not UV-resistant, you can put a UV film over it.  
They should have UV film to put over non-UV 
resistant glass.  If the matting is not acid-
free, ask them to replace it right away.

I really hope that they did NOT trim the cel to 
fit in the frame.  Also, I hope they did NOT dry-
mount the cel.  If so, say good-bye to your cel.

(As a side note, even with UV-resistant glass, 
the cel can still fade over time.  Excellent UV 
protection would require tinted glass, but hardly 
anybody likes the way tinted glass looks.)