> These attempts at limiting the content of the mail were upheld by the Supreme Court, but in the 20th century, the Court took a more assertive approach in striking down postal laws which limited free expression, particularly as it related to political materials.[7][8] The First Amendment thus provided a check on the Postal Power.

And SCOTUS these days also tends to equate cash as a form of political speech. Of course mailing cash is a bit different than mailing political materials but you could make the case that it’s part of a political organizing campaign or payment for political materials. And mailing cash itself through the USPS is actually perfectly legal as long as it’s for a legal purpose.