Guess I didn't describe my problem clearly because that didn't answer my question.
My Add to Cart button is not in the parent window but in the pop-up. The page I want to go to -- the Orders page -- is already open in the parent window, and what I want to do when the Add to Cart button is clicked is close the Add to Cart pop-up and return the focus to the parent window with the Orders page. But instead, the pop-up tries to open the Orders page in the pop-up window.
I tried using an onclick=window.close() on the Add to Cart button but eCart doesn't consistently add the selected item to the cart when the onclick tag is present.