I use the 60 hour packs, mostly because if a shipment is 24 hours late, a 40 hour pack will be expired by the time the shipment gets there. I have also noticed the 60 hour packs don't get as hot as the 20-40 hour packs (and I prefer it that way). I worry more about overheating in the warehouses and trucks and the first few hours in transit than I worry about them getting too cold. Since Las Vegas is almost always warmer than where they're going, I usually open the packs immediately before putting them and the animal(s) in the boxes, so that the heat pack, in theory, isn't producing much heat in the time it takes them to get out of town. I place them red-stripe side down to slow the amount of oxygen that gets to the pack, also in an effort to start it up more slowly (the red side is supposed to be the exposed to oxygen side).
Julie, if you still need them, I can overnight you as many as you want. Give me a call 702-658-5659 and they'll be there on Wednesday morning if that's not too late. High shipping charges may make this unrealistic.