How about:

"Dear stbx,
You're right. Why should you pay child support unless I agree to do what you want?
Next time you want to threaten me, send it to my lawyer."

Needed to get that off my chest.

Wonder if my taking the kids to see the new Bond movie after school tonight (discount movie night) had anything to do with his change of heart. He had to drop them off at the cinema and wait while we put their school bags in my car.

We always went to Bond movies together as a family - even to see the old ones on the big screen at special screenings. It must have made him feel sad. None of us even enquired whether he'd like to come see it with us tonight. The kids have asked him to join us every other time I've taken them to see a movie since BD.