Some of the NPCs offer "courier missions". These NPC missions offer a contract to deliver X amount of commodity Y to a random location. An offered mission is only available for a limited time, and after 20 or 30 minutes, you will be offered a different mission.
If you accept the mission, you have two days to complete it. To complete the mission, show up at the specified location with the commodity, and select "Active NPC Mission" from the drop down menu in the upper right part of the screen and press the "complete mission" button. These missions can be lucrative, and there is also an XP bonus paid upon successful delivery - however if you cancel a mission you accepted, you will lose XP and credits.