Unfair Business Practices

Mistakenly made a reservation with a manual transmission at O.R. Tambo Airport in Johannesburg but needed an automatic. Realized this after I got the car and asked to cancel the reservation so that I could quickly make a new one online through a third-party and get a decent rate. Hertz refused, insisted that I keep the original booking and charged me an extra $27/day for an automatic. In whose book would this seem fair and reasonable? I'm having my bank get involved to resolve this matter after the fact. What they should do is sanction Hertz for such predatory behavior.