I wear glasses for astigmatism, and based on my experiences with opticians, I would advise against buying them online. Here's why:
1. You need to try on the frames first to see if they fit your head, if the arms are long enough to go comfortably behind your ears, and if they just plain look good on you.
2. Any optician who knows what he/she is doing will measure your eyes before they make the lenses. They need to measure how far apart your pupils are so the lenses refract in the right place. (I don't know all the proper lingo, but I know my optician makes several measurements to cut the lenses right.)
3. When your glasses are finished, the optician will need to fine-tune the fit. He/she will adjust the arms, make sure you can actually see better, and that they got the prescription right.
I have never shopped for glasses online. As I have a family history of eye disease, I see an ophthalmologist and feel my eyesight is worth the extra money to get my glasses fitted right.
That being said, my hubbie wears reading glasses. The best glasses he ever found were straight off the rack at the nearest drugstore. He paid $10.00 for them, and they fit him the best and give him the magnification he finds most comfortable. Any honest optician will tell you that if a dollar-store pair of reading glasses work for you, get them--the magnification is what they are made for, nothing more or less.
Good luck to you. If you do decide to buy online, share your experience! I'm curious...