Help for module shipping

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http://forums.oscommerce.com/viewtopic.php?p=162274#162274
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Excused for my English 

I have added this code to the module flat1 :
function flat2() {
      global $order, $cart;

      $this->code = 'flat2';
      $this->title = MODULE_SHIPPING_flat2_TEXT_TITLE;
      $this->description = MODULE_SHIPPING_flat2_TEXT_DESCRIPTION;
      $this->sort_order = MODULE_SHIPPING_flat2_SORT_ORDER;
      $this->icon = '';
      $this->tax_class = MODULE_SHIPPING_flat2_TAX_CLASS;
      $this->enabled = ((MODULE_SHIPPING_flat2_STATUS == 'True') ? true : false);

      $totale= $cart->show_total();// this is new code
      if ($totale > 150.00){
	  $this->enabled = ((MODULE_SHIPPING_flat1_STATUS == 'False') ? true : false);;}// end new code

Cause an error in admin while in catalog is ok .

the error causat in admin is :
Fatal error: Call to a member function on a non-object in c:\programmi\easyphp\www\umbriastyle\shop\catalog\includes\modules\shipping\flat1.php on line 29
 while?   
Grazie Antonio