If this is a domestic segment operated by AA (or one of its express carriers), you can add the infant at check-in or even as late as the gate. There is no charge, only the requirement that the infant be "ticketed".

You may also call AA within 24 hours of departure to do this as AA has at this point taken control of your ticket. Prior to that BA has control of the ticket and will charge according to its pricing structure.

The examples above are unhelpful as LGA-YYZ and ORD-YYZ are international and accordingly charged as such.