Haven't read the other posts, so I'm not sure how much money you might be talking about. But there is no way in hell I'd agree to keep my mouth shut about her despicable breeding business. Gag order? Forget it. Go to court.
I wonder if you are not the first person she asked to sign a confidentiality agreement? It is aggravating she agreed to send the check and then pull this crap the day it is to be mailed. She deserves some well earned wrath.