I saw a very cool feature on a 1shoppingcart demo (don't worry I'm loyal to ShopSite), it was a post sale upsell - very slick.
They way it worked was after the customer hit "submit" to process the payment the shoppingcart processes the payment but before the receipt/thankyou page is displayed, the customer is present with an upsell window where the only option is yes/no. If yes is selected it is immediately added and the receipt/thankyou page displays the original item and the upsell item. If no was selected the original order is maintained and the receipt/thankyou page displays the original order.
This is a VERY effective way to grab an upsell, no "here's your cart, are you sure" just yes/no and then it's added or not and the best part is the original order is maintained.