Legal Question in Criminal Law in California

You need an attorney. You do not want to plead guilty to a misdemeanor. Assuming you have not gone to court yet, you do not know whether the District Attorney is charging the case as a misdemeanor or a felony. If it is charged as a misdemeanor, you are entitled to an appointed lawyer if you cannot afford one. I would suggest that when you go to court you ask whether it is charged as a misdemeanor or an infraction. If it is charged as a misdemeanor and you are financially eligible you can ask for an appointed attorney. Otherwise, you ask for time, and the judge will give you time, to hire an attorney. I suspect the courts are sufficiently crowded that an attorney can get you an infraction. Few judges or DA's want to go to a misdemeanor jury trial on a open container case.

As to whether you sign the proof of service letter, it makes no difference.

It sounds like you received a citation, which is a promise to appear, not a proof of service.

The actual code section that I am familiar with is Business and Professions Code section 25620, subdivision (a). A violation of that section is an infraction, not a misdemeanor. I'm not sure what is listed on your citation.