Yeah...pitch that idea to a postal inspector. Or a patent lawyer. Or a stock broker.
The postmark is a legal government act.
If you don’t think the Post Office doesn’t take that seriously, I think you might want to ask someone who knows. My Dad was a postal inspector.
Wow!!! Your dad has obviously been retired for a long time. We have had our mail diverted and stolen twice now for weeks at at time after people filled out fraudulent change of address forms. Do you want to take a wild guess at how “seriously” the Post Office took that? It took multiple phone calls, letters and emails to get it straightened out each time. We were never contacted by any postal inspector and they would not even tell us where our mail was being delivered to.
My step grandfather worked for the Postal Service for decades... it is not the same these days.
A postmark is one of the very easiest “legal government acts” to forge. No apperant effort has been made to make them difficult to forge. The link in the post you are responding to will make you a Postmark stamp to any specification that you want, with any font and design you desire for $15.95. It is not even a crime to possess such a device or have one made. And no the post office couldn't give a rat's behind if you are not using it with criminal intent. But even if you are... they would likely not be any more interested than the FBI was in Hunter's Laptop.