Generally, heat degrades just about every cosmetic product and our NOX is no different. If left in the heat for a few hours, it should not be an issue. However, if left in extreme heat for days would most likely degrade the product.
Vitamin C, the main activator ingredient in one of the chambers, is affected by heat and that is one reason why we say on the bottle to store in a cool, dry place. Nonetheless, a few hours in heat (ie: if left in the mailbox for a few hours after delivery) should not affected the integrity of the product.