Declare the Actor you want to use as your courier as a property:
Actor Property MyCourier Auto
You define which actor you want the property to point to in the Creation Kit.
You can then do what you wish with the Courier. But you will need an Event to trigger the script (search Events in the CK Wiki and see which one fits best your needs).
And if I may suggest some reading: http://tesalliance.org/forums/index.php?/topic/5039-class-2-properties/ It's a tutorial on how properties work and what you use them for.