Imo, a having a confirmation pop-up every time a user adds an item to the cart could be annoying.
I simply have a cart summary <div> on the page displaying the number of items and the total current value of the cart. When a user adds/deletes or changes quantities of items, the cart summary information is updated automatically.
The user also has a 'view cart' button which displays an itemised listing of the curent cart contents.
Anyway, just some food for thought.
If you really want a pop-up, all you need is an alert() box, a small window or another <div> with the confirmation message appearing when the code adding the selected item to the cart has confirmed the item was added. The user can click a button to close the pop-up or you can close it using setTimeout()