What does your ticket receipt say regarding checked baggage fees? That is what you will be charged

AA's ticket receipt state the cost for the 1st and 2nd bags

I would have thought that the extra bag fee could well be $150 given that you are doing both transpacific and transatlantic flights in the itinerary, but if the AA receipt says $75, then that is what it is

Don;t use the Qantas website information - its reference to codeshare flights almost certainly refers to QF codeshares on other airlines