One more problem with the accent

I had another incident which shows my lack of understanding of the Southern accent. I went to the local public library to ask about possible jobs. Since no one was in the administration office, I started talking to one of the staff members working at the children's reference desk and pumping her for information. After a few minutes of chatting she suggested I go to the circ desk and ask to see Miss Donna Davis. I couldn't quite make out the name because of her accent, and so clarified with, "I'm sorry. What was the first name? Mizahna?" "No, ma'am," she replied, trying not to smile at my mistake. "Here we refer to everyone as Miss or Mrs. followed by their name. I said Miss Donna Davis." "Oh, I see," I replied. "Thank you." I guess it all turned out okay except that I probably should have ended with, "Thank you, Miss." Or would it be ma'am? How am I supposed to know if she's married or not!! I thought I had passed that stage in my life where I have to check for wedding rings. Or is the Miss and Ma'am really based on age? And why are there two choices for woman when there's just one choice for men (mister)?