I looked in the logs and the most recent one says:

[09-Jun-2015 14:06:59 Australia/Perth] PHP Warning: include_once(includes/modules/shipping/flatau.php): failed to open stream: No such file or directory in /home10/xxxxxxxxxxx/public_html/store/includes/classes/shipping.php on line 58
[09-Jun-2015 14:06:59 Australia/Perth] PHP Warning: include_once(): Failed opening 'includes/modules/shipping/flatau.php' for inclusion (include_path='.:/opt/alt/php53/usr/share/pear:/opt/alt/php53/usr/share/php') in /home10/xxxxxxxxxxx/public_html/store/includes/classes/shipping.php on line 58
[09-Jun-2015 14:06:59 Australia/Perth] PHP Fatal error: Class 'flatau' not found in /home10/xxxxxxxxxxx/public_html/store/includes/classes/shipping.php on line 59