there are a couple problems here.
the big confusion, i think is work flow.
you cant add the item to the cart on the same page that you collect billing information
the flow of the site should be:
Add item to the cart by clicking the add to cart button.
show cart contents on a cart display page.
The cart display page has a link to the checkout page where the user enters billing information.
the way you have the page configured wont work, because the checkout button posts to the confirm page, this bypasses the add to cart code that you have on the checkout page.