I don't use cold packs as I have not found any that would remain effective long enough to be used for priority shipping. I just try not to ship if the daytime temps are over a tolerable range for the shrimp (about 85 or so). I try and check the temps where the packages will be shipped to. Also, my post office has two pickup times, early and late afternoon. I have been dropping the packages in the late afternoon as they are moved indoors overnight and it reduces the time they sit in a truck. Even dropping them for the evening pick-up, they are usually delivered within 2 days. I also insulate just like I do for cold weather in order to try and stabilize the temp in the box. So far so good!