I think this is true because when I printed out a Media Mail label using my Stamps.com software a pop up window came up and said something about if that the Post Office may inspect my package and if I am using the incorrect rate the Stamps.Com could be forced by the Post Office to close my account. This was the first time that ever happen so it must be something new. Also the Stamps.com website was down Friday and Saturday and when I used my account on Sunday to print the label that is when I got the warning.
It's not new. All but first class mail can be opened for postal inspection to insure the proper rate has been paid.
First class includes, priority and express mail and can not be opened
except by the Postal Inspection Service after they obtain a warrant from a federal judge.
Postal folks are quite paranoid about the sanctity of first class mail.
Maybe it's not paranoia though, the Postal Inspectors do watch and run sting operations.
Opening first class mail is a federal felony.