A key concept in Drupal Commerce is the Product Display vs Product model used to separate physical products from their display on the website. Depending on your point of view, this makes perfect sense or is a conceptual or practical nightmare. However you feel about it, understanding the reasoning behind the concept is essential when it comes to planning and implementing a Drupal Commerce project. http://nyccamp.org/sessions/drupal-commerce-product-vs-display-conundrum-and-how-explain-it-customer