I worked at the USPS for about 17 years. Nobody knows what's inside the ballots. It's illegal to open first class mail. You can repair them if they've been ripped open by the machines or other ways (one of my jobs was repairing damaged mail), but you can't open a sealed first class envelope. And frankly, nobody cared about absentee ballots. We just wanted to get the mail processed.
The only people who would open first class mail are the occasional thieves (one was fired where I worked shortly before I started). But the thieves certainly don't care about opening ballots....no money inside them. Trump is off his rocker....as usual.