UPDATE: We won!! Seriously, I feel like I just walked out of a courtroom with my head held high.
So she wrote back, and basically kissed my ass. Her whole tone was a 180 from her previous semi-threatening e-mail... here she said, "I agree with you and apologize for any anxiety I caused. As you know I fully believe in keeping good-faith business practices [which she mentioned in her previous e-mail, as a basis for asking me to pay $300]. Have a good wedding!" Yeah, go fuck yourself, lady.
Anyway, good riddance. And thanks for all your help, TFP'ers and especially my soon-to-be husband
(just over 3 weeks! yayyyy!)