Repair 1.5.5 admin invoice operation

Apparently in manipulating variables through the code, the admin/invoice.php file for ZC 1.5.5 referenced a non-existent class that wasn't declared.  Corrected this in this commit for ZC 1.5.5.  Appears that variable had been changed to reduce the need to declare a variable that already existed; however, the wrong variable was referenced in each of the invoice files.

Partially Fixes #31 for ZC 1.5.5. Need to address for other ZC versions.

1_5_5/admin/invoice.php

@@ -187,7 +187,7 @@ function couponpopupWindow(url) {
             //create array for use in zen_get_customid
             $attributes[] = $order->products[$i]['attributes'][$j]['value_id'];//value_id option_id
             //get custom ID
-            $customid = ' ' . $products_with_attributes_class->zen_get_customid($order->products[$i]['id'],$attributes) . ' ';
+            $customid = ' ' . $products_with_attributes_stock_class->zen_get_customid($order->products[$i]['id'],$attributes) . ' ';
             //only display custom ID if exists
             if( !empty($customid) ){
               //add name prefix (this is set in the admin language file)