I decided to return the car after hours downtown (Hertz downtown Perth allows this as their new location has a key drop) and then tax to the airport the next day. With the extra day (since Hertz is closed early on weekends) and the drop fee - the money was a wash without adding in parking. So no need to deal with the hassle, drop fee or potentially higher rate.

Turns out when renting a car from Hertz downtown and returning it there - rates in Perth are pretty reasonable