I am sure you are a very nice young man. Very nice, and also very young. But I'm mostly basing this on the sound of your voice, so perhaps I do you an injustice. However, let me just go ahead and share something with you. When your website has malfunctioned, causing me to lose an order place last Thursday for delivery Tomorrow for a wedding on Saturday; calling me "Dear" in that "calming" tone of voice is A-1 guaranteed to tick me off.
In general one should address professional women-- particularly customers, because in this economy, I doubt you can afford to lose even my small business-- as "Ma'am", or "Madam" or at the very least "Ms F." I would say that one wouldn't want to mistakenly address a young-sounding woman as "dear", just on the off chance she might turn out to be a 31-year-old professional, pregnant mother of three who is in no mood to be patronized.
the woman who got a little b!!@h-y with you on the phone yesterday morning