The trouble is because this part of your standard E-junkie button code:
...has been changed to this in your site:
The standard onclick event we provide in our code intercepts the button click to load the cart as an overlay, so if that event is removed, the button works like a regular link and loads the cart as a separate page.
If you didn't make that change manually, most likely some WP plugin you're using did that automatically. You may be able to use the Raw HTML plugin to prevent WP/plugins altering the button code; you can search for and install that plugin from your WP Admin panel, or read more about and download it from here:
If you did make that change manually, you can use both onclick events by appending them together, like so: