Actually, ordering shoes online for a killer price is a great idea...if you are already using the exact same shoe...

Otherwise, it's probably a bad idea if you've never tried the shoe and plan to rack up many miles. Heck, even trying a new shoe on in the store won't give you much of an idea how well the shoe will work for you. It's not until you reach about mile 5 of that first run before you might get an inkling.

Find a good shoe and stick with it for as long as that particular model is offered. I once bought a brand new pair the day of a 15k race (not recommended practice) and finished with a great time, and with happy feet and joints - they were identical to the shoes I already had.