Great question — answering as the UphillTech team:
I ended up calling Qatar Airways (+1 877 777 2827 for their US number, didn't have any wait when I called) and according to the phone operator, they told me they follow the price indicated on the Qatar Airways' extra baggage page . I only checked one flight ticket issued by Qatar Airways though (with one leg being a codeshare, not operated by Qatar Airways). The chat support however gave me some different information ( page 1 , page 2 ): Sure, let me clarify. If your ticket includes a codeshare flight operated by another airline, excess baggage can only be purchased at the airport check-in desk. The applicable charges cannot be confirmed in advance and will be advised by the airport staff...
Full answer by Franck Dernoncourt: https://travel.stackexchange.com/a/204699 (CC BY-SA 4.0).
Need it done for your business? UphillTech.