The customer is taken through a series of offers based on what you have set up in your sales funnel. After someone buys the front end product they will be taken to the upsell page. If they do not purchase the upsell they are taken to a downsell page if you have added one. If they do not buy the downsell they are taken to their purchase. If you do not have a downsell and the customer says no to the upsell they are also taken to their purchase. Customers will always be taken to the portal to pick up their purchases if they decline a downsell.