In my experience, sunglasses seem to get sat on, lost, left in Ops, left in the cubby next to the seat, left in seat pockets, dropped on the apron and cracked so why bother? Use the sun-shades, plastic cards, static paper; whatever and do without.
After 36 years of flying without them and still with the ability to see well enough to fly, save your money and have a good meal each few months with the money you save.