3rd Party Email signup on checout page

[kwikstand]

Does anyone know how to use a third party email signup during checkout? Right now, I have the standard check box for the customer to opt-in or out. Is there a way to send the email to a third party email list? For example: Constant Contact.

I currently have the Constant Contact sign up link in the right column of the product, category and home pages. But most sing ups occur during the checkout process. And it should be as simple as a check box, rather than requiring the user to re-enter his email address in yet another box.

Does this make sense to anyone?

PS: (I tried using the CC's signup form in the right column of all pages, but it broke my website for IE. Firefox and Chrome worked fine, but in IE, when a variable like color was selected, it would result in a page error)

Thanks,