I am creating a website and will only use wordpress for the payment/user management portion. To clarify, the main website will be 'www.xyz.com' and the wordpress install will be 'members.xyz.com'.
I want to be able to send the user to the checkout and add the right product to the cart using a URL query string parameter.
A simple link that can be added to any external website:
<a href="http://members.xyz.com/shop/cart.php?item=1">Download 1</a>