It should not be possible for your garage to reach a temperature near the melting point of our kayaks. Consider that plastic garbage cans, 5 gallon pails, and milk jugs are all made from similar plastic resin. While your garage may reach a temperature slightly above 100 degrees, it should never come close to 400 degrees. In general, high temperatures are more likely to occur in a vehicle with closed windows on a hot day where it is possible to reach 150 degrees. Even in that situation, if the temperature were to get so hot that it could melt our kayak, the dashboard and all of the other plastics in the car would melt as well.
The best thing to remember is not to have the kayak under force while stored. Often, conventional plastic kayak owners leave their kayaks strapped tightly to car racks on hot days in the sun and the kayak tends to form to the rack. Even when that occurs, they are able to remove the straps and wait as the kayak returns to its original shape.
With a Pakayak, there is no need to strap it on roof racks nor to put it under that type of force. It is part of the beauty of being a Pakayak owner!