Online orders
I'd only buy plants online if I had a large tank to stock, then I'd get a couple combo packs to save on shipping.
For a smaller tank, it's probably beter to go to the LFS and buy some cheap fast plants to break the tank in, then pick up a few cooler plants now and then as things get under control. Otherwise you may end up paying enough on shipping that you might as well buy local. Also, why pass up an excuse to visit one of the better LFS? I know of 4 good places for plants around me, and have only lived here a year. And that's not counting petco, petsmart and walmart. Check your local aquarium society webpage for info on local shops.
I like to see what I'm buying, then I can get swords that have stalks w/ babies and/or several plants in one pot.