Marketpress: Adding item to cart


I am creating a website and will only use wordpress for the payment/user management portion. To clarify, the main website will be '' and the wordpress install will be ''.

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="">Download 1</a>

Thank you!