I don’t know about you, but I lose the little microfibre sack that comes with sunglasses as soon as I take the glasses out of it. I put the case wherever, and it’s never seen again. So this idea is part magic trick, part gimmick, part practical. What we do is that we have that super fine microfibre sack attached to a tiny cord with a mechanism such that after you’ve taken the glasses out of the bag, and as you open the foldable glasses, it pulls the bag into the cavity in one of the arms of the glasses. When you fold it again you can see an edge of the case poking out of the arm of the glasses and can tug it out and wrap it around the glasses.
Take glasses out of case. Open glasses. Case disappears. Magic. There’s probably a bunch of physics getting in the way here. But perhaps if you paired with a professional magician they could build something like this — it is after all a very similar trick to all their others.