Cool idea! I think there are a few things on the usability side that you could do to make it even better:

* Lock the body when you open the modal by setting overflow: hidden. That will prevent people from scrolling around accidentally while filling out the form.

* This is related to the first point, but you should probably position: fixed the modal

* Make the label for the monthly donation clickable - people shouldn't have to find the little check box!

* Lighten the .iframe-subheader-text as it can be pretty low contrast when the background is white

* Add some subtle animations to the open and closing of the modal. It feels very jarring right now.