I should also add that Zenni’s additional options for the glasses are significantly less expensive than the competition. Getting thinner lenses (i.e. 1.61 or 1.67, etc.) or tinted lenses ($5) or Anti-Reflection Coating ($5) or photochromic or polarized lenses is significantly less than any other shop I know of, online or brick and mortar. (AR coating can literally cost about $50 alone elsewhere, tinting at least $20, etc.) And Zenni includes anti-scratch and UV protection free in all glasses.
They manufacturer the glasses in China, which is why it is so inexpensive. But the other companies also manufacturer their frames in China, although many of them still produce the lenses in U.S. labs.