Sounds like a mistake like Amazon had.MarineDepot has the Tunze 6095 for $15.00. Just bought three.
Strange, they are still showing it on website. I paid for mine and they will ship when in stock.
I think if they already accepted your cash its a done deal. I'm obviously not a lawyer but under UCC code they as a merchant are not allowed to back out. Good luck with it.
I'm sure there is some fine print somewhere which protects them in situations like these.