I live in Southern Ca where it gets quite toasty in the summer! If a cake is too big for me to store in the fridge then I make sure that I am frosting it the day before delivery and that there are no perishable fillings. But storing fondant covered cakes in the fridge shouldn’t be a problem. Any condensation on the cake will dry as it reaches room temp. Just make sure not to touch it or you can leave fingerprints!