As long as the item you're shipping fits correctly into the flat rate box without bulging the sides or other parts of the box including the top when you seal the box then it doesn't matter how heavy the item is as long as the box can handle the weight without tearing through the bottom.
The top must seal flat with the tape without bulging and as long as it fits perfectly then the item will ship.
Just make sure that the box is not gonna tear with something too heavy for it.