Extremely Unpleasant Car Rental Experience

The car rental company charged 50 AUD for cleaning a child seat, which is highly unreasonable! When returning the car, I checked the vehicle thoroughly inside and out with the staff, and they said everything was fine and asked me to sign. Before leaving, I confirmed again with the staff that the car had been inspected and there would be no charges to my credit card, except for the highway tolls as those could not be billed immediately. However, after I returned to China, the car rental company sent a photo of the child seat, claiming it was dirty and demanding 50 AUD for cleaning. This is completely unreasonable! Why wasn't it mentioned on the spot when both parties were present? We knew beforehand that there would be charges if the seat was dirty, so we were very careful while using it and checked thoroughly before returning the car. There was no dirt at all, and the staff had also confirmed this. Now, after returning to China, they send a photo claiming it was dirty and demanding payment. This is very frustrating! Firstly, I checked and confirmed the seat was not dirty when returning the car; secondly, the staff also said everything was fine; thirdly, a photo alone is not convincing; and lastly, any issues should be addressed on the spot when both parties are present—claims made afterwards are not acceptable.