They can regulate the mail, they cannot regulate the voting. If they would deliver envelope A, then they must deliver envelope B, they cannot reject it simply because it is a vote.
Do you have no concept of the law?
The problem isn't in law regulating the postal service, it's in the FACT that multiple people handle each ballot, unseen and unsupervised during its processing. From mailing out to receipt at the counting office, that ballot is who-knows-where. There is little proof it was filled out by the intended recipient. Anyone handling it could tamper with or even dispose of it with virtually no way to catch them in the act.
THERE IS ZERO CHAIN OF CUSTODY OF BALLOTS IN AN ALL MAIL-IN ELECTION SYSTEM.
Now, you can claim there's little or next to no fraud in that system, but you have no way to prove that. You have no way to know if there are operatives out there offering cash for your ballot or anything else. There are now videos of political operatives offering the homeless in LA $5 to register to vote. Let's say come election time--and they have weeks to do this--those same operatives offer $5 to $10 to the same homeless to get a blank, signed ballot from each one, no questions asked. Those operatives then fill out those ballots and mail them in. The supposed voter signed it. How would you know it was fraudulently cast, which it now is?
Or ballot harvesting is legal. A political operative goes through a neighborhood trying to collect ballots for turn in. They flag each ballot they think is going to vote against what they personally want voted for that they receive. They might even deliberately go to neighborhoods that typically vote against their positions. They then take those ballots and shred them rather than turning them in. How would anyone know? Ballot harvesting is legal. The voter handed it over legally. The harvester, out of sight and out of mind, destroyed the ballot or held it to the day after the election before mailing it. You have no way, ZERO, to detect that election fraud.
Mail-in elections are absolutely rife with potential for fraud.