WooCommerce has an option to run various UI actions via ajax.
If you are developing a theme and want to update your own elements during Woo ajax calls you will want to add the following hook:
add_filter('woocommerce_add_to_cart_fragments', 'my_woocommerce_add_to_cart_fragments');
function my_woocommerce_add_to_cart_fragments($fragments) {
$fragments['#my-dom-element'] = '<div id="my-dom-element">Some new markup with some computed value.</div>';
return $fragments;