I was wondering if you could help me with the code that i should paste on promotion template in order to make a overlay popup window? I need to make a popup window come up when a customer puts a specific product to the cart (to let him know that if he buys an extra item, he will get a discount). Thank you very much in advance, Anna
For starters, there is an Amasty extension that provides functionality similar to what you describe: Free Gift with Purchase.
If you cannot find an extension that suits your needs, doing this manually is definitely feasible.
Detect that product was added to the cart. 1) Use an Observer event, or 2) inspect cart on page load with jQuery (this is not standard).
Reveal a template of your choice. For example, templates/custompopup/popup.phtml. Include it in the page with layout XML, or potentially your Observer could add the block programmatically.