I just heard back from Unbounce and here is what they said:
Unfortunately taking a look into this shortcode, the issue appears to be that it does not seem to be valid HTML format to embed to the Unbounce pages. Because this is third-party code from s2member, we would not be able to troubleshoot this directly not having written the code ourselves. We would recommend returning back to your resource for this snippet and request a chunk of valid embedding HTML code for your ClickBank shopping cart buttons.
It is valid HTML what the button outputs. I don’t know how Unbounce interfaces with WordPress or what is the problem with using the shortcode. The thing is that the ClickBank buttons have to use the shortcode, not static HTML.