The link below are to some fold-flat sunglasses that I picked up once at a MOMA store for $12.
http://www.flatspecs.com/Sunglasses.aspxThey are a bit costlier but they fold totally flat and come in a rigid case. I keep them as back up shades in my Jeep's emergency kit.
The drawback to them is because they fold flat, they don't bend around your face and leave large un-protected gaps on the sides.