this is basically the gamble anyone takes when ordering from zenni/other online retailers - the measurements and adjustments that the opticians make at the eye doctors office create a custom fit for your glasses. When you order a pair online, a lot of this process can’t be performed to the same standard. For most people with low/relatively normal prescriptions, this doesn’t really make much of a difference. But for those with larger amounts of refractive error, small adjustments can make a big difference and it can be hard to get that right online.

yes, the prescription is what allows the lenses to be custom to your vision

I fully support easier access to glasses, but prescriptions ensure that people get the proper correction for their visual needs, which needs to be done on a custom basis. There’s an argument to be made for having prescriptions last longer, but eye exams also provide valuable insight into the health of a persons eyes beyond their need for glasses. IMO the problem here (as with many healthcare issues in the US) is a lack of access and coverage for important healthcare services.