Check that you are downloading:
zen-cart-v1.3.7.1-full-patched-07052007.zip

And that the version on your usps.php shipping is:
* @version $Id: usps.php 6529 2007-06-26 00:15:57Z drbyte $
There is no mention of the Staging server in this:
PHP Code:
    $db->Execute("insert into " . TABLE_CONFIGURATION . " (configuration_title, configuration_key, configuration_value, configuration_description, configuration_group_id, sort_order, set_function, date_added) values ('Which server to use', 'MODULE_SHIPPING_USPS_SERVER', 'production', 'An account at USPS is needed to use the Production server', '6', '0', 'zen_cfg_select_option(array(\'test\', \'production\'), ', now())");