Of all of the thousands of different items I had to audit when working for HMCE, by far the biggest profit margin I ever saw was on spectacle frames. Naming no names - of course - but what you were charged maybe £99.99 for by -------------- actually cost them less than £2. Is it any wonder they can afford to look so generous when it comes to "second pair" deals?
I've used the same small independent opticians for nearly fifty years, except for the one occasion when "work" sent me to get some VDU specs to S--------s. I was robbed blind, so to speak, and "work" only stumped up a fifth of what they'd promised. Funnily enough those specs never helped me see the word "mug" in neon lights across my forehead.
You live and learn, I guess.