Whats wrong with sending a ballot via UPS?
Moreover, it's a completely neutral service - a ballot from Berkley, CA is treated identically in terms of priority as one from Amarillo, TX.
In a world where UPS gets to serve however they see fit, it's perfectly legal for the company to prioritize one over the other in terms of prompt delivery or to prioritize care that the mail arrives unmolested.
The USPIS needs probably cause to open your mail, as it's protected under the Fourth Amendment.
Meanwhile at Fedex or UPS, the little old nosy lady at the counter can bust into any package she finds "suspicious". And you have no recourse.
I'm not sure how many people realize this.
Well sometimes a lotto game ends up with a flaw that allows an arbitrage opportunity - you can fill out every combination and walk away with a few hundred thousand or a few million. A small group of people spend incredible efforts, seemingly irrationally, to fill out every possible lotto combo...
Now imagine that lottery is for president of the United States.
Effectively if you can intercept and alter ballots, you only have to change the 'right precincts' to flip an election.