You're hurt, and I fully appreciate that. Yet as a lawyer I have to advise you lawsuits for defamation rarely go anywhere, and few if any competent lawyers would handle such cases -- particularly on a contingency basis. Based on the brief facts you set out, the person merely stated that your "satisfaction guarantee" was a scam. She didn't say you or your company were crooked. You acknowledged that the rest of her statements were true. And you quickly deleted the posting from your website and made a refund, so the matter is almost certainly over and done with. Why give it new life?
Were you to find her and file a lawsuit, you'd be necessarily repeating the allegations she made, and giving them far wider and more durable distribution. She's likely post it on other sites. And news media might pick it up. Thus folks who never saw the original posting would read about her claim, and some would think it's true and assume you're just trying to bully her. That's not a good thing.
While there are several "reputation management" firms that purport to be able to help you improve your firm's reputation, I have some doubts as to their effectiveness and cost structure, and if her posting is gone, there's nothing to manage by way of damage control.
