What about leaving neutral feedback because they did attempt to fix the mistake? Neutral doesn't hurt their rating, it's just that - neutral. Something like, "Sent wrong poster, but refunded money." (I would wait until you actually get your refund though)

Then if they respond to your neutral feedback then you can respond - ultimately, it looks bad on them, not you. I know I don't buy from people who leave snarky comments after people have left them negative feedback I'm sure others feel the same. I just don't trust them.

Good luck!