It depends on the temperatures. In most cases, there probably is no difference but hot days can be lethal riding around in the back of an overheated truck. As an example, I sent a shipment out last week to a city where the low was in the 40s, thus needing a heat pack. At 3-5pm (pick up times) here at my location, the temperature was in the mid 90s so the back of the pick up delivery truck would probably be in the 110s. 110s + heatpack would most likely be lethal. Even if it wasn't, there's no way I would subject an animal to that heat in an enclosed container! By dropping the package off at 7:30pm, the temp was already in the mid-low 80s and quickly dropping to a comfortable and safe temp even with the heat pack.