It really just depends on the weather along the route and if the eggs freeze or not. You definitely want to pick them up at the post office unless your mail person comes early in the morning. The less sitting around in cold places the better. If you do Tennessee to Florida, you'll likely be fine. Not much of a chance for a big blizzard to trap the mail for a day or more.
You would want to avoid a bidding war online in the winter with eggs that would travel long distances, keep the cost down and cross your fingers. I bought eggs semi-locally when the weather was gorgeous, but when shipping time came, it dropped dramatically, the post office took 4 days on a 6 hour trip south so they sat around who knows where, but I have development showing through candling so I can't complain.
It's all a risk.. buying from someone you've never heard of, from chickens you've never seen, seasonal hazards of shipping, shipping itself and trusting the treatment of the box to who only knows. Then when you finally get them... you have to hope there isn't a serious storm that knocks your power down for days, or temperature fluctuation in the incubator, or anything else that can happen during incubation. This is why sellers don't guarantee a hatch, there's too many things that can happen. Their job is to get fertile eggs to you, nothing more. If they don't develop at all, that doesn't exactly mean they weren't fertile. There are a lot of reasons why a fertile egg doesn't start developing.
