Small bug in admin/includes/classes/authnet_order.php line 28.

You have:
include DIR_FS_CATALOG . DIR_WS_ADMIN . DIR_WS_LANGUAGES . $_SESSION['language'] . '/authnet_order.php';

You want:

include DIR_FS_ADMIN . DIR_WS_LANGUAGES . $_SESSION['language'] . '/authnet_order.php';