Isn’t delivery of mail one of the feds constitutional duty. More so than the hundreds of other things the fed gets involved in...
Yes it is mandated that universal service be provided, bit it does not specify frequency so Congress could eliminate a day or two of service.
If US postal service goes away imagine the Teamsters delivering voting ballots.
Not a duty of the government. But it is an enumerated power that was granted to Congress, as per Article 1, Section 8:
“The Congress shall have power ... To establish Post Offices and post Roads”
If Congress chooses not to exercise that power, then I would imagine that private industry will pick up the slack.