Unfortunately, for people with even moderately complicated prescriptions (e.g., even mild astigmatism), there's no substitute for in-person fitting, as the lens centre must be precisely located.

You just have to measure your interpupillary distance with an accurate ruler. It's very easy with either a mirror or a friend's help.

With a moderate astigmatism you need to center it vertically too, and that's not really possible without trying the frame on and seeing how it sits.

Does that number change over time? Sounds like a one-time calibration, pretty much like my jeans size.

No, it varies with each frame and lens shape. My eyes are offset vertically and I have severe astigmatism and it usually takes more than one visit to get proper fitting.

EyeBuyDirect and FramesDirect (both owned by Essilor) will remake your glasses with correct pupil/fitting height. Order the first pair, adjust them for correct fit, including pantoscopic tilt, then measure the vertical position of pupil. Make any compensation needed for tilt, then return the frame for a free remake with the new value.

When you order the same frame again, you can specify your custom fitting height.

Or walk in to an optometrist and have them competently do all of that for you and then sell you the glasses.

There is broad variability in competence of individual opticians, whether independent or at a chain store. If you have access to a competent local optician in your city, wonderful. If you don't, there's no reason to pay 10X prices for worse service than you can obtain yourself with a few minutes of measurements. Unlike an optician who needs to fit a wide variety of customer faces, you only need to fit a single face, which is a one-time learning exercise.

We meet again, Mr. Bell. I think you might have missed the last bunch of replies I gave you, you might wish to check!

There's one other thing those online vendors can't give you:

Wavefront aberrometer guided glasses. Unfortunately the industry sits at a shitty place here. Nearly all doctors don't buy those. And even if they do, their measurements aren't portable to the vendor systems. So instead, opticians have those. But opticians can't use cycloplegia, which you do need to get a proper wavefront measurement - and even if you can arrange that (say, by hurrying to your optician very quickly after a doctors visit), you'll run into the problem that most opticians don't configure those devices correctly, leaving them at the vendor default of 3 measurement cycles (instead of the 5 it takes to get statistically useful results), don't check for ring defects on the scan, and so on. It's a mess.

The two that I know of are:

http://zennioptical.com (which I've used for years)

https://www.glassesshop.com/ (which I learned of recently)

If we're sharing websites to obtain glasses, eyebuydirect has been my go to for the last few years.

Warby Parker, while significantly more expensive, does have pretty high quality frames with flat rates on lenses.

If your prescription is less than -5.00, Eyebuydirect provides free aspheric CR-39 lenses, which will be higher optical quality (especially for computer use - less chromatic aberration) than the polycarb lenses used by Warby Parker.

Many online eyeglass retailers are owned by Essilor or Luxottica, not to mention other parts (labs, equipment) of the optical supply chain.