Notwithstanding, the problem HAS NOTHING TO DO WITH ZEN CART. It's custom code that's having conflicts as a result of how your server is configured vs the code you're running.

The fact that you're running it inside an iframe which appears inside a page of your Zen Cart site is entirely irrelevant. Something running from an iframe is completely separate, and subject to its own interaction with the server.

Best to get support for that code from the author who wrote it.