The Pony Express carried mail in relay fashion across the American West in the 1860s. This Pony Express envelope is a primary source because it is an original document from the 1800s. What makes it a primary source besides the date? (Look at the stamps and handwriting to help you).
Remember, primary sources are objects and first-hand accounts created by witnesses who lived during the time of the event in history. Secondary sources are objects from a later time period and second-hand accounts by individuals who did not witness or experience the events as they have recorded.