High temperature means that all the food you just bought from supermarket is a ticking time bomb for a harmful bacteria growth.
Of course that number will start to drop once you open your windows or crank up the air conditioner. However, inside your car can be more than warm for nasty bugs, which can potentially make you sick and start flourishing on your food.
How long do you have?
Heat is a good medium for bacteria growth, so you want to minimize it as much as you can. The less time you have between shopping and going home, the better.
It is also important to shop and pack up your groceries. In store, get into the habit of keeping the final part of your shopping to the meat and dairy aisles to ensure that those items spend the least time in your car.
Once you reach the car, put your bags inside the passenger area, even if you do not use the AC, it will still be infinitely cooler than your Furnace of the trunk. Then head straight home and get everything into the refrigerator or freezer as soon as possible.