Ethiopian Airlines has a generous baggage allowance for international flights in Economy of 46kg (2 pieces x 23kg for the hold and 10kg carry-on). If you replace one of your 23kg bags for the hold with your bike box and keep your overall total to under 46kg, you might not be charged any extra bike carriage fee, though you will only find this out once at the airport. The standard quoted fee is $100 each way. When checking in for your international flight to Addis Ababa, you should try to get your bike (and other hold luggage) tagged all the way through to Mekelle, so that you don’t need to collect any luggage on arrival in Addis. Note that the baggage allowance for domestic flights is 20kg for the hold and 10kg carry-on, but this does not apply if the domestic flight is a direct add-on to an international flight.