WTF powers do the Constitution and statutes give the US Postal Service or federal govt to usurp state authority and create their own, independent voter roles?

None, zero, zilch, nada. But that's power John Roberts and his radical radical Catholic S.Ct. majority just gave them..

In the first instance, this will likely restrict mail-in voting, because the USPS will not forward mail-in ballots to voters not on Donald Trump's independently-created voter rolls.

But that's just the beginning.