I got one of those years ago and never replied. Never heard back again.

A friend got one some time after that. He got scared and got a lawyer. He phrased a response letter with a reduced payment. He explained to me, that the phrasing is very important. You cannot under any circumstance show guilt for the alleged infringement, because then they come back with a vengeance (demands in the thousands of €). Paying the 900€ would already equate an admission of guilt.