recorded delivery vs second class

recorded delivery

noun
  • A form of postal delivery where the sender receives a receipt of postage, and the receiver must sign upon receiving it 

second class

noun
  • A class of postage with delivery times slower than first class. 

  • A class of travel, inferior to first class. 

  • The second-lowest rank in the Boy Scouts of America, between tenderfoot and first class. 

How often have the words recorded delivery and second class occurred in a corpus of books? (source: Google Ngram Viewer )