Oliver tries to pick up a special delivery letter that arrived for him at the Hooterville post office. He and Lisa end up traveling all over the valley because Drucker mistakenly gave it to Hank Kimball, who gave it to Fred Ziffel, who gave it to Arnold. Arnold dropped it in the mail, sending the Douglases to the Pixley post office where two sets of identical twins are working at the windows. (thetvdb.com)