What is the most long lasting ice packs brand for shipping?

It's hot summer and I have very hard time trying to keep my ice pack cool inside the package that I ship my axolotl with. USPS priority express doesn't do all their express shipping for next day delivery when it comes to some out of the way locations. One day shipping became 2 days and my axolotl suffer for it ( most of the time they stucked in local office for extra night) I need to find the ice pack that will last longer than a day. Right now I use
ice-brix. By polar tech industries. They are moist control , safe, non-toxic , colder than ice ( that's what they claim) But in this heat summer it's no match no matter how I double my insulation with thick seal styrofoam box , lining with double reflector bubble seal. All taped air tide seal blocked like I'm packing for space travel. 2 days still too long for my ice pack to last :nono: I need to find better ice pack quick please help!!!!!!! :errr:

What you need is bulk ice in a bigger container. You can make your own by freezing water in plastic bottles, filled about 80% full and squashed so there is no air bubble and they do not burst on freezing. You can make the temperature lower by adding salt (or commercially various other chemicals such as iso propanol) but for animal transport I would avoid this as you risk freezing the animal and you do not need low temperatures. The only way to get a longer lasting ice pack is more ice!

I put in abou a pound of ice packs per square foot . Anymore than that it would be too cold for axolotl .