Pony Express vs post office

Pony Express

name
  • A 19th-century mail service delivering messages across North America by means of riders on horseback. 

post office

noun
  • A party game involving the exchange of kisses. 

  • A place (building, office, shop, or counter) concerned with the business of delivering letters, post or mail and selling stamps, etc. 

  • An organisation that delivers letters. 

How often have the words Pony Express and post office occurred in a corpus of books? (source: Google Ngram Viewer )