>and it does seem to be the case that they've been up front about their religious affiliation (online) at least since 2013, when I stopped looking.

If they were only soliciting funds on their website, which made it clear that your donation was being used to send 17 and 18 year olds to Israel, that would be a different story. In reality, the vast majority of their donations come in from people who are totally unaware because they hear the radio jingle, which is sung by little kids, and makes no mention of their religious affiliation or their affiliation with a foreign country. Here in New York I've been hearing these radio ads on a daily basis for literally decades and had no inkling about the true nature of this "charity" until today.