I've never had issues ordering glasses from Zenni and I think Warby Parker too, you can just put in whatever values you want. Contact lenses on the other hand are regulated, I have to get those from overseas without a prescription.
IIRC they basically try to chase up your eye doctor for you, and if your eye doctor doesn't respond within a certain timeframe, they have a greenlight to proceed with the sale according to a California law that was passed maybe 10 years ago. It's probably different in different states - maybe in some states you can literally just type in whatever you want.
Zenni does let you buy "whatever you want".