Is "post to express" about using the mail system? Using "post" as a verb to mean "send via mail" is typically done for British English but not American. An American speaker would say, "I am mailing a letter" or "I will mail a letter." To express that you are using express mail, you might say something like, "I am using express mail" or "I need to send this express."
You probably should not use "ain't" in any professional or academic environment, particularly in the Northern parts of the United States or anywhere where British English is spoken.