I use Uniheat heat packs
You can get them in 40, 60, and 72 hour varieties. I always prefer the 72 hour because in the event something does happen the fish will be warm for 3 days at least. However, I will not ship priority unless it's warm enough because there's not a guarantee that it will arrive in 3 days.
And thanks :) Alex was a gift for my Mom (which is why I was packing him in the first place). Unfortunately he died a few months ago, I believe he had a tumor but I'm not entirely sure because I wasn't there to look at him. My mom was devastated. I've been trying to find another red dragon for her ever since.