Browser back button stops working after user selects variant

I’m using shortcodes to embed the products in my site. Here’s an example of one:

[wps_products title="Foundation Daily Detox? with Joint Support" excludes="images, title, pricing, description" add_to_cart_button_text="Add to Cart" show_featured_only="true" items_per_row="1"]

My client has observed (astutely) that after a user selects a size on any given product the back button on the browser stops working. Upon a quick inspection in browser I do notice the warning: Empty string passed to getElementById(). when the user makes a size selection—though this doesn’t seem to be the sort of problem that might disable the back button, I though it was worth noting.

The page I need help with: [log in to see the link]