MarketPress: Buy Now Button Shortcode with Product ID Not Working

I'm using Marketpress with the SimpleMarket theme and I am trying to use the "buy now" button shortcode on posts and pages with a specific product id. The button shows up on the post or page and works when you click the "add to cart" link, but the specific product does not show up in the cart. It appears in the cart as the title of the page or post with no price. It is to my understanding that the product id is the equivalent of the post id, correct? How can I add an "add to cart" button on posts or pages for a specific product in the store?

Thank you!