[Plugin: WP e-Commerce] Add to Cart not working in IE

I am currently working on a number of issues with an existing site utilizing WP-ecommerce: https://www.natashashealthnutcookies.com

The most significant issue is around the Add to Cart functionality. Currently, a user of IE, who clicks on “Add to Cart” button on Order Cookies page, the pop-up asking whether user wants to “Continue Shopping” or “Checkout” does not actually get displayed.

This issue does not rear it’s ugly head when viewing site using Chrome or Firefox.

Please help.

https://www.ads-software.com/extend/plugins/wp-e-commerce/