I'll be a contrarian here. If I give to a charity and they have any way to get my email address, I expect to get their newsletter. This is not really spam, I contacted them. The key is the opt out, not the opt in.

Remember, charities have a brand to defend. While they are willing to spam to a degree, they don't want people angry at them. It is in their interest to limit the mailings and they generally know this.

The only caveat is that the charity should not exchange lists with other charities or buy names. If they do, don't let them have your product (if that's possible). Opt out is most vital then.

Phil