Intentionally delaying a mailing is already a crime in itself. And if you already have the power to shut down a mailing hub for a day or two without abyone noticing, I don't see how it would be more difficult to change the date in the stamping machine.

I have no doubt that, when the Nazis come to power here as well, they will use all measures to suppress free voting. We of all people know how that works. But in that case, using the stamping date for mail-in voting won't stop them anyway, as that's a simple law, that can simply be changed by the people in power. Case in point: This very article about changing the postmark application rules. It seems, they didn't even have to change a law for that, just some USPS internal guidelines.