Of course it's possible to make a mistake to book under a corporate rate by accident. The problem is that National may not see it this way. Most rental car companies are being strict with regards to using corporate codes and rates due to the high amount of abuse.

With regards to the charges, you will have a very hard time getting anything reversed. It really depends on the type of manager you speak to at National. Sometimes, you will get a friendly manager who will be willing to waive some of your charges. But, in most cases, they will not waive any charges. They will claim that it was the renter's responsibility with regards to booking the reservation properly.

In the end, this was a case of a rep taking advantage of someone who didn't have the proper knowledge with regards to renting a vehicle. Is it wrong in our eyes? Of course. Is it wrong in National's eyes? No.