Upsell Now provides you the option to close the popup whenever there's an engagement (add to cart/close button).
1) Go to Offers and click edit (pen icon)
2) Enable this option
Note: By default, this is set to off. The reason being is that some merchants would want their buyers to add more than one upsell product to their cart. Enabling this will prevent that behavior.
That's all! Happy Upselling!