Baggage overcharge or BAU?
 

Last month I purchased a mixed-carrier itinerary that went SFO-LAX-SVO-TAS. The SFO-LAX segment was US operated by UA, the rest was SU. Before the flight I called up US Airways to check on the baggage policies for such an itinerary and was told that the overwater carrier rules apply, even to domestic flights, i.e. the baggage allowance and the fees are determined by SU.

I had 2 bags to check in, and n the morning of the flight when I checked in at the United counter, the agent said I have to pay $100 for the second bag, according to US Airways baggage policy. The Aeroflot fee is only $50 for the second bag. Due to baggage cutoff window, I decided not to argue at the time and paid the higher fee, then followed up with United, who reiterated that the US Airways baggage policy applied, and not Aeroflots.

Where can I seek clarification on this? US agent told me one thing, UA tells me another..

You can appeal for a refund, but I'm not sure how far you'll get. In the future, I'd recommend when asking these types of questions, to do it by e-mail so that you have your answer documented and you have proof. When it's by phone, it's much harder to prove.

I thought the originating carrier determined the baggage policies/ fees?

+1, even for Int'l itins....

According to United's baggage page: "If your trip is marketed by a carrier other than United, that carrier’s policies apply." So it's the originating marketing carrier, not the originating operating carrier. In this case, it's US-marketed, so US baggage policies apply. (This is in accordance with US DOT regulations.)