Please--your race has nothing to do with it--not even in MN. Toba Garrett would never take a second's treatment of this kind. I would also bet that 80% of us did some self-learning on cakes before we got a chance to take classes or a course. You do not need a storefront, that is so much more $$$.
Make sure you have a good online presence (I bought a used digital camera on ebay for that). Set your prices according to your minimum wage and real cost of ingredients and rent etc.
You are probably hearing from people who do not pay more than Wal-mart for the simple reason that they cannot afford anything better. Please start looking at the suburbs of Minneapolis instead of the city centre for those people who know that they should pay $120 for a birthday cake. Go to your public library and ask to see census data for income. That will show you where to promote your products.
This couple--send them a letter telling them "I regret that I cannot meet your requirements". NO further details. Send back the deposit less the tasting fee. Make sure you keep a photocopy for yourself.
At events, a chefs coat is a good start. Make sure your hair is neat because that is the place that a lot of people look. Stand straight and don't let anybody tell you that you are out of line. Make sure you know your local health rules and that you can explain to anybody who asks, how you meet them.
NEVER NEVER NEVER consider to do work for anybody who asks you your race. That is ILLEGAL.
No you don't have to say a word, merely send a one-line email "I am unavailable for your event". Always cc yourself.