I think your eyes are too important to buy glasses online. I have had the doctor over correct once and it was easy to get it fixed for free because I bought the glasses from her. I am all for cutting costs but not with your eyes.
My prescription has been between -8 and -9 for several years now. I buy new glasses every 2-3 years, and it's vital that the glasses are correctly made . . . with the prisming that I get, when things are even a tiny bit off they seem OK for a couple hours and then tend to lead to nausea.
The past four pairs of glasses that I've ordered (all from different local opticians) have all had errors or mistakes made that needed correcting. Sometimes I had to take the glasses back more than once before they were corrected. None of the shops around here do the glasses in house, they are all sent away to a lab to be corrected.
To prove that I'm not crazy I get an appointment with my optometrist and have her determine if the focal point is in the right spot on the lense, and measure the prescription. Then she writes me a note indicating corrections that should be made.
I've been considering buying online with a place with a good return policy because of the poor quality from local opticians. My eyes are too important to keep buying glasses locally. :P