My wife and I recently took a vacation in Ethiopia.
We travelled independently but purchased our domestic flights through Blue Sky Ethiopia Tour and Travel.
I would thoroughly recommend this company. They promptly replied to all my emails and were incredibly helpful. Buying the domestic flights through them saved me A LOT of money!
Most importantly, they were more than happy to book the tickets even though I wasn't taking a tour with them (i never had a reply from other opertors).
The tickets were delivered to my hotel in Addis (I paid cash on collection). They also arranged a mobile sim card with credit for another $10.
I'm very happy with their service. A++