I'm an optician. If you get your PD, you need to get your Monocular measurement. That's for each eye. Your Binocular PD may be 66mm, but in reality, your right eye may be 32mm, and your left eye is 34mm. If you have a high RX, that number is very important. Being off by 2 or 3 mm can induce prism.
Also, I know it sounds like the optician is whining, but it really is a liability issue when giving out that number to someone not buying at your location. If the doctor is not measuring it, it's not a true part of the prescription. This comes into play a lot when I work with parents wanting that number for their kids, ordering glasses for the first time online. The PD measurement does change for people under 18. People's faces get bigger and wider. A mom gets that number when her daughter is 3, and thinks it is the same when she is 7. That number could be 10mm different by then.
Folks ordering single vision lenses online don't have as much of an issue. But you need to be very careful ordering a multi-focal lens, like a progressive online. You need to be measured correctly for the seg height for the bifocal. Your PD may not change once you get older, but the seg ht will always be different for every pair of glasses, and especially a metal frame with nosepads.
I'm a very frugal guy, and am always trying to save money. I've ordered a few pair of online glasses for myself and have been happy with the quality. But I just want to play devil's advocate since I'm in the field and deal with this stuff every day. There are many different options for lenses too, so if you are shopping around, you need to compare apples to apples. There's not just one progressive lens, or one "thin style lens". It's just not all about the "greedy doctors that want to steal all your money."
In a lot of cases, the actual RX can come back incorrect from where you buy it. You get them in the mail and just wear them for a year or two. Meanwhile, the RX was slightly wrong and you weren't aware because you adjusted to them after a bit. Again, this is a big problem with kids, because kids don't always know how to tell their parents they aren't right.
Also, proper measurements by the optician are key. Another issue I deal with all the time is someone that got their RX from my doctor, then took their RX to Costco or Lenscrafters to have the glasses made. Then they get the glasses, can't see, go back to where they bought them. The place checks the actual RX, tells the patient that the RX was made correct, and to go back to the doctor for a recheck. Patient comes in, I check the RX. It is correct, but the measurements are incorrect. Once everything is lined up properly, then the pt can see.
This is tough to do when you get them online. Now if you are only paying 30-50 dollars per pair, then it may be worth it to play the odds.
If you have any questions about anything, feel free to send me a PM. I've been doing this for over 12 years. If you are trying to figure out what to buy based on your RX, let me know and I can give you an honest opinion.