You're not oversensitive, SB... I agree with all of you, it's totally rude! Wow, I've never heard anything like that before!

Haha it'd be funny if you sent a gift instead. Like something silver for the house from Tiffany, or something nice like that. That way you're acknowledging that you got the note, and you're helping celebrate their silver anniversary, but your own way, not like a charity contribution. Actually I started that out as a joke, but the more I think about it, I think that's what I'd do.