There are, at least in theory, pretty serious federal laws in place to prevent tampering with, or delaying, the mail.

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.

This is most important imo. We are granted certain protections with USPS that will likely never exist with private carriers.

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.

You've heard those stories where the odds in a lotto get lopsided right?

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.

I'd imagine UPS (a courier) is a fair bit more expensive than USPS (a national postal service)?

The USPS is subsidized to keep prices low, and furthermore is a public institution which seems to be a clear benefit for mail-in ballots.

The USPS receives no government subsidies and for example pays property taxes. There are a lot of government mandates about what they can or can’t do, but it doesn’t currently receive federal funds.

cost and ease