Of course Congress risks a potential Constitutional Crisis in that mandate. The USPS is one of the only services of the federal government that is Constitutionally mandated. The contradiction of Congress wanting USPS to act as if it were a for profit business remains an interesting balancing act versus the Constitution.

(It's also not a surprise that the Congressional mandate to run the USPS as a for-profit has been used as much to make the USPS profits a slush fund of "free money" that Congress can use for non-USPS projects, including pork barrel projects. The for-profit mandate isn't just for threatening the shutdown of USPS services.)