opentaps offers a Cross Selling feature that appears on the Shopping Cart screen in the box called "You Might Also (sic) Interested".
This box will suggest, in a short list, some randomly selected items from the same catalogs as the Products already in the Shopping Cart (that is the Line items already in the Cart).
If items are grouped into the Catalogs with this feature in mind, such as by including mostly the items very likely to be of interest to a shopper who is looking at one of them, the effectiveness of the feature may be optimized.
For example, items of slightly higher, and slightly lower positioning might be listed together, along with the most popular accessories for those items. This would be similar to the approach taken in some of the best known retail ecommerce sites.
Product_Promotions << Manual Pages >> Shopping_Lists,_Quick_Add_List,_and_Shopping_Cart_Persistence