$(document).ready(function() {
  $('a[rel="external"]').click( function() {
      var window_width = ($(window).width() - 800) / 2;
      var window_height = ($(window).height() - 600) / 2;

//      var email = $("input[name='wc_email']").val();
      var shipment_number = $("input[name='shipment_number']").val();

      if (shipment_number) {
        var myWindow = window.open($(this).attr('href') + shipment_number +'/', $(this).attr("title"), 'width=800,height=600,left='+window_width+',top='+window_height+',titlebar=1,toolbar=0,scrollbars=yes');
        myWindow.focus();
      }

      return false;
  });
});