From a FOIAed Department of Transportation document on investigative techniques:
A “mail cover” is the process by which the U.S. Postal Service records any data appearing on the outside cover of any class of mail, sealed or unsealed, or by which a record is made of the contents of unsealed (second-, third-, or fourth-class) mail matter as allowed by law. This “rnail cover” is done to obtain information in the interest of protecting national security, locating a fugitive, or obtaining evidence of commission or attempted commission of a felony crime, or assist in the identification of property, proceeds, or assets forfeitable under law.
Seems to be the paper mail equivalent of a pen register. I’d never heard of the term before.