Oh, here's a quote from their website on having to do it in person!

BUY MILES

ShebaMiles membership offers you the opportunity to buy up to 20,000 miles within 12 consecutive months. You can buy a block of 1000 miles for $ 25.00 plus a service fee of $5.00. Blue, Silver, Gold and Platinum members are eligible to purchase Miles. Purchase miles at the nearest Ethiopian Airlines office and your miles will be credited within 2 days. Just a couple of notes: Welcome members cannot purchase miles. And Purchased miles will be credited as Bonus Miles to the member’s account and are neither counted towards Tier qualification nor can they be exchanged back for cash. Finally, all unredeemed purchased miles will be subject to expiry as per the mileage expiry rules of the program.

https://shebamiles.ethiopianairlines.com/#/buy-miles