Try contacting the manufacturer or their hotline at 1-800-232-7677, Monday through Friday, 8 a.m. to 5 p.m., CST/CDT.
What another mom answered about storage conditions in the warehouse is true. I worked for a drug manufacturer and warehouses, trucks and UPS do not have special temperature controls for products stored at room temperature. Deliveries are made year around including winter.
I use Pediasure for my child, stored sealed at room temperature, and refrigerate it after opening it. The temperature of the car (38 F) is the same as a refrigerator and I'm pretty sure it is OK. On the other hand, if the temperature was above room temperature (more than 76F, like a hot car) for a prolonged period most medications would start breaking down and then it wouldn't be OK.
But I understand your concern, to be safe call Similac and ask.