After reading everyone's responses, each time I got a sick feeling in my stomach. We do not need to come down to this type of person's level... the simplest response is to be prepared as professionals should be. Always know the exact place you are to bring your cake before dragging it around to every table/room in the place... ask to see the designated cake receiver person before you bring it in... have them sign off on the cake delivery... and get out of there. You have filled your contract.
Now, this particular woman was totally not professional... you should simply have asked, "And you are?..." Your response should have been, "i am sorry, but you are not designated by the bride to have any opinion. This is her wedding and this is how she contracted this portion of it".
Then, you ask to speak to the manager of the place... keep your composure and take care of the matter at that moment. Do not wait hours, days, weeks, and write letters. Take care of the problem immediately.
You simply tell these ego-driven people that you wish to speak to their superiors and will not leave until they come. This saves grief for everyone... plus it provides a learning lesson for the wicked witch that this is unacceptable behavior from any human being and you are a going to give her a loving lesson in how all of us should act.
This will come back to bite you if you get someone in trouble... it always does. Simply pick up the phone and call the manager of the establishment and let him know, in your most professional voice and without the kids screaming in the background, that you had a unfortunate situation with one of his employees. You ask for an appointment with him and his employee to get the matter straightened out so it will not happen again.
That woman may be the only support for her family and maybe she was PMSing that day... I am not on her side, but as professionals, we do not have the right to come down to these people's lack of social graces.
It is time we all quit screaming to get a situation solved. A generation ago we would not have had this problem... it is time to return to civility.
I can tell you, without a doubt, that I have never run into a person who acted like that for very long, because I look down my nose at them and ask them their qualifications for trying to attack me in such a horrible manner. If you state you can see they are having a "horrible day", they will back off of whatever they are trying to do to you.
Be the better person. Go into the place and meet with them eye-to-eye. You will always get the upper hand, and you will establish yourself as a true professional... not a raving lunatic.
Sorry, I think all of you are wrong for wanting to write letters. Confront the problem head-on and quit hiding behind the pen.