Got a response from MainWP:
In the wp-e-commerce\wpsc-shipping\library\shipwire_functions.php file, the command in the issuee is isset( $_POST[‘server’] );
This function should be updated by the author of WP ecommerce. As a temporary solution, you can try to replace the function with the following:
public static function is_active() {
return ( (bool) get_option( ‘shipwire’ ) && function_exists( ‘simplexml_load_string’ ) ) || (isset( $_POST[‘server’] ) && !isset($_POST[‘mainwpsignature’]));
}