Pop up promotion when you click on Checkout!!

I thick there should be a way that when the customer clicks on "Go to checkout" it pops up with a promotion to encourage them to put something else in their basket. For example I have seen this on the McDonald's ordering system, when you have completed ordering what you want and click on checkout it pops up and says "would you like to add a double cheese burger to you order?" This always gets me! I think this would be a nice little up-sell feature.