There seems to be some confusion over whether you can always get a discount on internal flights if you book via Ethiopian Airlines, and how to go about getting them. Most advise that you should book while in Ethiopia and show your international ticket to get the discount. This may not be the most convenient if you only have a short time in the country.
I found that if you send an email inquiry to the booking office in Ethiopia after you have your international ticket booked, they will refer you to your local office (in this case Washington) who will book your tickets at the discount. I booked a 3-leg northern route with a list price of $480 for $210.