$900 dollars in damage?

So about, four months ago I guess, I got in a small accident. Seriously nothing. I scraped some old lady’s car, barely at all, and apparantly made a dent in it (although I couldn’t really see it at the time when they pointed it out), with the bumper of my car. Mind you my bumper isn’t made of steel or anything; I drive a volvo and the bumper is sorta like hard plastic I think.
So I get a letter today that reads:

We have written you on prior occasions… we are writing this final letter… you have 20 days to reply.

I never received any other letters! And they want $846.88! The damages COULD NOT BE that much! I am outraged. I barely scratched her **** car; I should have just driven away.