But that is irrelevant .
The fact is it that the postal power is authorized (like the military is). And thus appropiations are legal.
These things are by their very nature are NOT "economical ". Which is not to say that they should not be run economically.
These essential and authorized powers were understood by the founders to be inherently unprofitable. And as such it was understood that they would have to be funded by the power to tax/tariff.