Hi – any food – even unopened shelf stable foods – are sensitive to storage in heat. As you say, temperatures in cars left in the sun climb much higher than outside temperatures. Ordinarily you should never be storing ANY food in sustained temperatures of more than 25°C.
I live in the tropics so even driving home from the store I have had fresh foods spoil on the journey if I get stuck in traffic. I learned from that never to put the groceries in the trunk.
There are many variables that contribute to it, but you could say that for a powdered food like Huel, its shelf life reduces by an average of 30% for every 10°C increase over 25 degrees.
The coolest part of your car is the footwell behind the passenger and drivers seats. So the best option would be to keep it in a cheap polystyrene cool box in that area and if possible put a small bag of ice in there whenever you get the chance to buy one from a store. Park your car in the shade when you can and if it’s safe to do so, leave a window or sunroof partially open to let hot air vent out of the car.
