It's best to treat your spray tan solution like you would treat a pharmaceutical product.  You wouldn't keep your toddler's medicine in the boot of your car, so don't treat your solution the same way.  Solution is best stored between 4 and 20 degrees.  A great place for small amounts of solution is a fridge that can go on low - but not too low!  Alternatively look for cool dark places away from hot water heaters and air conditions.  The bottom of a dark cupboard, or an area within the garage might well suit.