I have sent mirrors before and the best way is to make a box out of ply nearly the same size as the mirror and a whisker higher than the mirror.
Fix a small angled wedge in each corner and place the mirror up side down on these wedges, don't wrap in anything.
Screw the lid down, don't nail, with some cardboard to keep it firm, and also some scrap cardboard around the edges to stop sideways movement.
Leon
