I haven't ordered from Anime-Cel.com in a long 
time, but I used to in my early days of 
collecting (I just haven't seen anything I 
wanted! I swear!) ... ^^;  Anywho, they use (or 
used) a really flimsy piece of cardboard.
One day I came home, expecting to see a package 
on my doorstep, and was horrified to find it 
crammed into my tiny mailbox!  Maybe the postman 
was having a bad day  O.o;  The package was 
folded, but somehow the cel escaped damage.  
Maybe it was because the cel itself was rather 
stiff with paint... maybe it was a miracle.  All 
I know is that I _love_ the way Anime Shop Vi, 
Anime Chaos, Usagi's House, Animanga, a lots of 
other dealers ship  ^_^