Zen Cart Checkbox Cross Sell

Checkbox Cross Sell allows your customers to add additional items to their cart when buying another item, merely by ticking a checkbox. It's like Impulse Buy for the Product Info page.

Here's what the product info screen looks like with Checkbox Cross Sell: 

Only specific products (i.e. not categories) will be presented as cross sells. Items are selected from your Better Together configuration as follows:

• For prod_to_prod linkages, if you are on the page for the first product, cross-sell the second product; if you are on the page for the second product, cross-sell the first product. 
  Example: for add_prod_to_prod(A, B, "$", 50), if you are on the product info page for A, show B. If you are on the page for B, show A.
• For prod_to_cat linkages, if you are on the page for an item in the category specified by the second item, cross sell the product. 
  Example: for add_prod_to_cat(A, B, "$", 50), if you are on the product info page for an item in category B, show A.
• For cat_to_prod linkages, if you are on the page for an item in the category specified by the first item, cross sell for the product. 
  Example: for add_cat_to_prod(A, B, "$", 50), if you are on the product info page for an item in category A, show B.
• Note that twoforone_prod linkages are currently ignored.

The item will then be shown under the following conditions:

• The item is not disabled (out of stock or otherwise).
• The item may be added to the cart (i.e. if you search for the item, the search results page shows the item with "Add to Cart" functionality and not a "More Info..." link).
• The item does not have attributes or the item does not have non-defaultable attributes and you have the Default Attribute module installed.

To emphasize this point again: items with non-defaultable attributes cannot be displayed as cross sells by Checkbox Cross Sell. 
(Note: Products with defaultable attributes may be used, if you have installed my Default Attribute mod.)