The upgrade offer is likely for PHX-CLD which is a short ca. 1 h (wheels up/wheels down) flight on E175. On such a flight you will be getting drinks and packaged snacks from a basket. Not worth of $100 for sure. However, leaving MIA you should be able to get into the Flagship lounge even you are flying AA with AA status - as long as your connection in MIA is <24 h. Safe travel!