Hi,
Do you hold any status within oneworld? as all oneworld Emerald, Sapphire and Ruby members do not pay for baggage charges on AA regardless of the fare.
If not, American may not charge you as you have an international sector as connecting to/from an International flight.
If AA do charge you then it will be taken when you arrive in the USA as you will have to re-check your bag (after customs) plus AA will have to re-issue your boarding pass, so you will have to go to an AA desk.