I've been hesitant to respond to this for a few days (partially due to the rather emotional reaction my etiquette faux pas has garnered from some people here), but I do feel it needed to address a few things that were brought up by other esteemed posters.
Several of you are questioning my mother's decision to not open on Mother's Day. While it is perfectly reasonable that you may be surprised to hear of a florist who is not open that day, please remember (as Blithe said) that your local customs are not universal. One poster stated that he or she lives in NYC and has never heard of a florist closed on Mother's Day. Neither I, nor my mother, live in NYC. In the city she lives in, it is more common to see a small business closed on Sunday than it is to see one open. This is a cultural throwback to the time when our state, by law, forbid businesses from being open on Sundays (with a few exceptions). Those laws have since been removed, but the culture of closing on Sundays is still HIGHLY prevalent in our area. Therefore, knowing that, it is more of an oddity to hear of a customer from our town who is upset or surprised by my mother's decision to be closed on Mother's Day (or any Sunday). A business in our area saying "we are closed on Sundays, even Mother's Day" is about as remarkable as a coffee house advertising "we sell coffee." It is a highly expected, run-of-the-mill thing in our town, and not really even worthy of note to most people.
A few people have suggested that I should have referred her to a competitor in order to help her find someone who can deliver on Sunday. But, as I did state previously, "having grown up in the floral business in that city, I do know that none of the florists in town deliver on Mother's Day. There are about a dozen florists in the city, and all of them know each other and work well together, referring business to each other (based on need, location, and what particular skill each has). Had there been a florist in town who delivered on Mother's Day, I would have referred her. But, alas, there isn't."
Second, I have fully admitted, several times, that my actions were out of line. Perhaps they were brought out due to my emotional feelings toward the designers (my mother, my sister, a young lady I find to be quite delightful and endearing, and two women who were like mothers to me). Or, perhaps, it was my young age and brashness that caused it. Either way, I fully believe that, although the customer was a "special snowflake" through and through, I did act inappropriately. I do not deny that.