I go to the local flea market: good quality polarized sunglasses that fit perfectly over my bifocals are $4. Use a string to tie them on (better than Croakies). Buy 2 pair, one to wear and hang up on the rearview mirror, another pair for someone else to sit on or scratch by jamming them into a gritty glove compartment. Solar Shield brand is good = $20.

Also- as previously stated here- lick the dang things off and spit out the salt. My $400 plastic bifocals last many years when licked off. NEVER RUB ANY PLASTIC LENSES with a nasty cloth or shirt.