How to Create Upsells that Boost Your Sales
Upsells on a smaller level can easily be done on your shopping cart page before the customer gives the credit card information and closes the purchase. In this case, the upsell is positioned at the top of the page, visible to the customer as they land on that page. The customer can take a moment to look at what it is, decide to click the check box, and add it to the shopping cart or else say no. The purchase continues from there with filling out shipping and credit card information to complete the purchase.
From our very good friends over at entrepreneur.com