NEW 72 HOUR UNIHEAT SHIPPING WARMERS / HEAT PACK other supplies at www.snlivestock.com/store These are an absolute must have for shipping EGGS and CHICKS in cold weather. (works with Reptiles,insects,tropical, fish and plants as well) The packs heat up to max in about 4 hours and heat lasts up to 72 hours. These have been used with hatching egg shipments and they are priceless!!! It works great with priority shipping, FedEx or UPS that usually delivers within 3 days. The box still needs to be insulated in some manner for these to work. Further, they should be located as low in the box as possible since heat rises, and a small hole needs to be put into the box where the heat pack will go so that it can breath properly and circulate warm air through the package. Last but not least, these heat packs will not work if they get wet. So, if you are shipping to where there is snow on the ground, leave them a little bit off the bottom of the package. Surface temperature of up to 100°F. Increases internal box temperature up to 10°F higher than outside temperature. Lasts for up to 72 hours. In some instances, this is more than the entire transit time of your package. Simple to use!!!