MD: Don't do it. I would only consider buying frames on-line and then have my OD do the lenses. For one thing, they have to measure and mark the focal point of where your eye actually looks through. for them to center the lense right. No way to do that over the 'puter. Also, a better OD will have the best quality lenses available and I am a big believer in quality materials.
It's also like Quicksilver stated. Service and support is there with a local guy and that counts for alot.
BTW, my F-I-L is an OD.