My opinion is obviously not going to be popular but if this is a dear friend, I wouldn't have charged her. If I know I couldn't do it without feeling resentful I would have told her I couldn't do it at all.
BUT, because you did tell her up front what it was going to cost, I don't like how she is handling it either.
It sounds like she and her dh are much more hesitant to put out that much money for someone that has caused so much stress for them. (The stepmom that is.)
I think a good compromise would be to give her the 10 photos for $150.
All that said, friends and money don't mix. You said she has used you before and paid with no problem, and now look. You got a problem. Bummer.
I tend to agree with the person that said she doesn't want to feel taken advantage of by the stepmom. NOT taken advantage of by you. I would try to give her the benefit of the doubt on that one. She sounds like she is caught in the middle here and what you've said about her I really don't think she's trying to shake you down. That is the problem with email, texts etc. It is hard to read tone and uncomfortable to ask for clarification.