I agree with Lilro--screen all communication and ignore as needed. She has absolutely zero right to contact your son--she isn't even married to your ex, is she? And it doesn't sound like she's reaching out in ways that are hard for you to manage. Just throw that mail in the trash! I do think you should also talk to your son about the situation, if only to encourage open communication between you and him, but at some point you are also the mother who knows best.
I think often the simplest solution is the best. File those letters in the circular file and go about your day. Why get involved in negotiation with her? You owe her less than nothing, and him exactly nothing! No need to justify, accuse, defend or explain. Go about your business and leave their insanity to them.