Regulations and Procedures: Bill

So my parents got a letter stating that there has been a mistake, and were being charged the wrong amount for the past three years (they were being charged less than what was owed). Now they want them to pay the difference for the three years owed (this is a city in Southern CA). Can they actually do that? It's their own mistake, and now they want to get money out of no where. Going to go, and talk it out as soon as possible. I just want to come prepared. Any advice or information where I can find a defense like a law that prohibits them from doing this would be very helpful. Any advice will help.

If wrong section if it can b e moved to the right section thanks.

Regulations and Procedures: Bill