Is this to use the magento crosssell function to display a particular product(s) at the checkout of another unique product in the normal way? The extra feature required is to hard code a promotional price or % discount into the crosssell item. If a sku 123456 is in the product view or category it has a retail price but if show in crosssell it is shown with a promotional discounted price that is controlled at product level. So maybe this feature is part of the product setup, maybe it's a new field called crosssell price ?
I have asked aheadworks for clarification on the function of their 'checkout promo' extension.
The ahead works, extension seems to be a banner but I'll see what they say.