Warby Parker (https://www.warbyparker.com/) makes some really nice "luxury" glasses, even though they still cost less than the cheapest pair you'll see in a typical store. They'll even send you 5 frames to try on at your home.
If you want a specific frame for some reason, it's a good value. I bought a pair of Oakleys from glasses.com ( a luxottica online outlet) for a savings of about $50 from retail. I think the high index lenses were cheaper too. But if you buy $8 glasses, they are going to be junk.
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They are not "cheap plastic frames" by any stretch. I'd put them up against the Luxxotica frames any day. They use the high-end plastics, with metal-reinforced stems. The Zenni's I tried, on the other hand, have no reinforcement, and the plastic is definitely more brittle.