I had never thought about what happens if you buy online and have a defective tire, until I saw a review on Tire Rack from a really pissed off guy that had to remove his defective tire and send it back, then wait to hear if they would warranty it! So his car (had a space saver spare) was then sitting without a wheel/tire for 2 weeks while they screwed him around. He then got to pay for demounting, remounting, and another balance. F that. I never bought online tires again since I saw that.
Every time I needed tires since, when I took a little time and price shopped, I would be VERY close to the online prices, a few times slightly below, once you added mounting/balancing fees.