This one scumball is mulm out the drain; you just have to get past it. Believe me, the $ isn't worth the mental headache.
If you're going to be selling over the internet, I would suggest that:
Before you put up a listing, have one or more links to independent pages that provide care and feeding instructions; have the links in your offer. (I mean, really, a cooler pack even if it did get to 65 F is a little weak.)
Clarify your DOA policy as suggested above.
Always print out an AccuWeather copy of the weather prediction for the week for the buyer's ZIP code before you ship animals. If you have a similar issue with weather after the fact, you can get unarguable AccuWeather or US gov't (Dept of Commerce I think -- maybe FAA) numbers for high, low, precip, etc. for every day in his area code. (You can get it hourly but I think that's a subscription service from AccuWeather, and 6 - 12 months after the fact for the Feds.) Practice this so you don't have to learn under pressure.
Nil desperandum. As you've said, the vast majority of people actually try to be decent folks most of the time.