You would need to include the http:// in the &postto= URL, like so:
http://www.e-junkie.com/ecom/oplug.php?ej_stamp_pdf=true&pdf_allow_copy=y&pdf_allow_print=y&postto=http://mysite.us4.list-manage1.com/subscribe/ejunkie-ipn?u=a0411f46773831dfe91f7d0ed&id=04d6b1XXXX
However, I don't think the &postto= parameter can handle URLs that have their own query parameters, such as the ?u=a0411f46773831dfe91f7d0ed&id=04d6b1XXXX at the end of that URL.
Fortunately, if you want all your buyers subscribed to the same MailChimp list, you can just put the MailChimp URL in your Seller Admin > Account Preferences > Common Notification URL, then the product's own Payment Variable Information URL would just have the PDF Stamping URL. The Common Notification URL receives the order details for every order, regardless of the particular item(s) ordered, while a product's Payment Variable Info URL only receives the order data when that particular item is ordered.