I don't have any experience or insights of it, but the very first thing that came up to my mind is: frame glasses. I've heard multiple times already that price

I don't have any experience or insights of it, but the very first thing that came up to my mind is: frame glasses. I've heard multiple times already that price margins are huge at that one, and nobody are determined enough to offer lower prices for them. But that's only what I've heard from here and there, I'm not into that topic "that much" to confirm that - OP asked for some suggestions, so I gave one ;)