| Nonstop Flights: Austin to St. Louis $120-$150 r/t [August] (midweek) â American Airlines American Airlines has round-trip flights from Austin (AUS) to St. Louis (STL) for $120-$150, NONSTOP. Flights August. Mostly midweek. Main Cabin. Not Basic. A Carry-on bag, personal item, and seat selection is allowed for free on American. Click for more details and booking links Nonstop Flights: Austin to St. Louis $120-$150 r/t [August] (midweek) â American Airlines . | 