Please, sorry for my bad english, but I need help with this:

I used during online booking registration code 706900, cdp 20% off.
But many people say me, that this code works only during online booking, but when I´m get pick the car, Hertz staff canceled my car with this code and they give me new car, with highest rate. Please, is this true?
So, what now, with this registration? Cancel? Or risk?

Again, sorry for my terrible english....

Many, many thanks for all....