Mr. Feasel is a former prosecutor in San Mateo County (CA) with over 10 years of criminal law experience. Nothing stated on this site shall in anyway be construed as legal advice, or as creating any attorney/client relationship. If you would like to hire Mr. Feasel to further investigate your situation, feel free to contact him thru this site.
The civil demand letter does not create a separate obligation to pay unless followed by a civil lawsuit which almost never happens.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Ignorning the civil demand letter is just like ignoring a bill. You can not be criminally prosecuted. If you have a criminal case you are facing feel free to contact me for a free consultation. Good luck!