I decided to only check-in one bag with AA, which weighed 23 kg, so the 8 kg excess baggage for INR 1600 works. I now understand why Indigo has the first tier as 8kg for excess baggage for international connecting passengers. 15 kg domestic + 8 kg makes it 23 kg check-in bag for international travelers.

I found Indigo to be better than Air India for excess baggage rates. Air India does not give a preferential rate for passengers coming in via an international flight through a non star alliance airline and/or on a separate PNR.