From Longman Dictionary of Contemporary Englishsecond-class mail/post/stamp etcsecond-class mail/post/stamp etcrelating to the system of delivering mail in Britain that is cheaper and slower → second-class
Examples from the Corpus
second-class mail/post/stamp etc• The quantity relative for second-class stamps is 140.0, indicating an increase in numbers bought of 40%.• First-class and second-class mail should be put through the machine on separate runs. 