Dear All,
I don't know if anyone else has been having this issue, but when a Customer is logged in. After adding a item to there cart, and then viewing the cart with shipping estimator showing on the shopping cart page. The whole right column side-boxes and footer do not show on the page and if you look in the error logs you get the cryptic message
Fatal error: Cannot redeclare class order in /****/****/public_html/includes/classes/order.php on line (usually this is the last line of the order.php file)

After many hours of looking on the forums here. I finally did the bandaid fix (as DrBytes said) of changing the
PHP Code:
require(DIR_WS_CLASSES . 'order.php'); 
in file shipping_estimator.php to
PHP Code:
require_once(DIR_WS_CLASSES . 'order.php'); 
I do not know why the issue is in the latest version(s) of Zen Cart, but this does fix the display issue.
I hope this information helps you too.
--
Red Baron