I hate to say this, but you seem so naive about cake business ..
How are you gonna make a profit and stay in business if this is how you will handle these customers ? Sounds like you are giving away free Wedding Cakes. I'm sorry about what has happened to you, but I think they took advantage of you and you probably fell for it. I am on your side and we all are, but you have to stand up for yourself and do the right thing, otherwise you will be at loss. Why in God's name did you return the check ? after you gave her the cake ? If the cake was terrible, bad and disgusting, then I can understand why you would just let her step all over you. But they ATE it !!