Cheapest places to buy + install polarized, non-prescription LENSES?

I have a nice pair of Givenchy aviator frames with no lenses. I want to have polarized, non-prescription sunglasses lenses installed in them. I called a few optometrist around me in the prices range from $100-$250. The cheapest I found was $90. Are there cheaper online services?

 

I get my glasses online from a site called 39Dollarglasses.com. Cheap eyewear starting at $39 but of course you can spend a lot more if you want a nicer pair, I just get cheap glasses because I wear contacts most of the time.

Anyway, I know they do polarized lenses as well..maybe they could be worth checking out