Tak to zatim zkousim resit takto, na client-side mam nasledujici script:
var itemCount = function() {
$('span.shoppincart-item-count').load( stylesheet_dir + '/cart-count.php' );
};
$(document).on('click', 'input.wpsc_buy_button', itemCount );
promennou stylesheet_dir definuji na zacatku kazde stranky
<?php
$stylesheet_dir = get_bloginfo('stylesheet_directory');
echo "<script>var stylesheet_dir ='" . $stylesheet_dir . "'</script>;"
?>
akorat nevim, co presne dat do cart-count.php, protoze zatim mi to vraci prazdnotu
<?php
printf('%d', wpsc_cart_item_count());
?>