This made me laugh as we've just done something similar to my SIL.

She has 5 cats, and we all tease her about being the Catlady whose house the kids are afraid to approach. While she was here over the holidays, one of my boys put a magnet on the back of her car with a picture of a psycho cat, which read "Want to buy a cheap cat".

She called this morning when she found it while loading her car at the market. She immediately knew it was us, of course, and I was glad we gave her a giggle.

I like the idea of acknowledging people's kindnesses this way. Thanks.