I just went into BAEC account to manage my reward flight booking. Then, I tried the choose a seat option which provided the AA booking reference number. I used this information to manage my booking on AA website. Once my flight was found on AA website, I entered my Passenger information which includes your frequent flyer program and number. After filling all passenger information, I returned to chose my seat and the cost of selecting a main cabin/preferred seat is now ZERO.

Thank you for your help and suggestions.

"Membership does has its privileges"