US carriers, including AA, do not charge for infant tickets on domestic flights. You may handle the infant ticket at any point up to and including the gate. Given all the other hassles associated with infant travel, why not call now and have AA add the infant ticket to the existing PNR and associate it with your other tickets and be done with it?

Note that this is a wholly different situation from the international situation where AA would charge 10% of the then available F ticket for the infant.