Add this at the bottom of cart.liquid to auto-save your cart attributes

gistfile1.html

<script type="text/javascript" charset="utf-8">
//<![CDATA[
jQuery(function() {
  jQuery(window).unload(function() {
    var params = {
      type: 'POST',
      url: '/cart/update.js',
      data: jQuery('form[action="/cart"]').serialize(),
      dataType: 'json',
      async: false
    };
    jQuery.ajax(params);
  });        
});  
//]]>
</script>