I don't have any cold pack to test, but what I will do is place the package in my car. Then check later the day to see how high temperature is. Everything will be as is with the package except no fish. :) I don't know how hot usps trucks get in the trunk, but I'm sure it can't be as bad as a car with sunlight shooting through directly. So if this test pass (temperature < 90, it will be a success assuming temperatures that day is 100 or more. You'll be surprise with how well insulation can hold up to it's name. :P I'm very excited about this, hopefully I can get this done by Thursday. If this fails, I will definitely consider cold packs, and also test that to see if it will drop temperature below 70 or if it even keeps the water safe for fish, 2 days minimum to be reasonable, or however long the brand claims to keep cool. This will have to wait after the first experiment though.