We recommended a heat pack for winter shipping when temperature is in the 40s F. In our experience heat pack does not work when temperature is below freezing. We would hold shipment when temperature is not at least 32 F. We also recommend hold orders for pickup at either Post Office or Fedex Office. Most of the damage occur during transporting in unheated truck during delivery.
72 hour UniHeat heat packs (shipping warmers) activate slowly, up to 3 hours. Their maximum temperature is similar to the 40 hour heat pack. 72 hour shipping warmers reach their peak temperature in approximately 24 hours. 72 hour UniHeat shipping warmers are the best choice for 2nd to 3rd day shipments.