Hello, Jane,Welcome to Just Answer and thanks for your question. Marriage records are not available online for New York. You need to apply giving as much information as you have, and they will search the records for you. Here is the site to get you started: http://www.health.ny.gov/vital_records/genealogy.htm The fee is quite modest ($22 for a 1-3 year search time frame to $62 for 11-20 years). You will pay a professional researcher $35-50 per hour for their time plus expenses. They would pass along the New York fees to you as expenses.
You are right that Charles is a common name - not just in that area of New York, but everywhere. It would be pure, blind luck to run into someone who can trace their family back to Margaret and Andrew Charles.
I would order the marriage record and then decide what to do.