Don't have much knowledge into the efficiency and the economics of government services, and you do have a valid point. But would the millions of elderly that rely on it for their medicine, SS checks, etc., still reliably receive their mail without additional cost to them or the sender if they were shipped through a private carrier? The suspicion and outcry for the changes to the USPS is clearly a timing matter. Why does Trump all of a sudden care about the postal service?
Lets be real here for a second. Trump simply doesn't want mail-in voting because he knows that it will result in more democrats voting and him likely losing the election. Every decision he's made about the USPS has been with that concept in mind. He even said in an interview on Fox news last month that if mail in voting is allowed you'll never see another Republican in office again. He knows it. Everyone in politics knows it. The right will never admit they know this, and will look for any and every excuse to hold back the USPS without directly saying that its because they don't want more democrats than republicans voting.
With mail-in voting being accepted in a majority of states, and even set as the primary method for a few with COVID present, it really screws things up for the GOP. Couple that with Trump's gamble that convincing his voters the virus is no big deal will get them to the polls, while the scaredy cat democrats will stay home. So he spent a very long time building up this giant "virus hoax" to calm his masses. He is simultaneously trying to train them to believe that mail-in voting is fraud, even though he, himself does so, as have many, many of his voters, historically.
It worked. But what he didn't account for was states creating blanket acceptances of mail-in voting applications. That instantly turns the tables, because not only does it undo the removal of polling places, but it also allows the democrats (and the GOP, if they so choose) to vote from the safety and comfort of their homes while avoiding lines, saving time, and not risking COVID. His supporters, however, will be going to polling places in full force, in what will likely be a very bad time for the country as COVID starts to rear its face again between flu-season and school openings (especially in red states that are in denial and have entirely in-person classes).
So then, what happens, is a ton of his voters are going to be against voting by mail, and most will have not applied to do so, forcing them to go to the ballots, in what will most likely be a time that the pandemic is gaining speed. His advisors basically laid out the future for him of his silly comments, and where they were leading. So, in recent weeks, you've seen him compliment "absentee voting", trying to making it a completely separate thing from "mail-in voting", as not to look like an incompetent liar to his voters. But he knows his advantage still lies at the polling places, so he is simultaneously trying to hinder the USPS from delivering ballots. The number of mail-in voting applications for democrats FAR surpasses those for republicans, so while it may cost him a few votes, he is betting that it will cost democrats so many that his supporters who do show up will be able to tip the scales. He is also taking the approach of preparing to call the election invalid due to the amount of mail-in voting, should he lose.