It would be cheaper for the reason that when you weigh the parcel in the post office. It would somewhat float thus the weighing machine that sets the price would give you a smaller reading, thus cheaper price. Alas in real terms you are not saving money, well you are the post office is losing. The reason is this. If you consider the parcel being sent by airmail. While on a scales the package would be lighter the airplane is carrying the weight of the parcel and the air around it. Thus even if the package floated in mid-air in the cabin. It would be the same as if it was tided down.