Critical error when notifying customer with status and comments

[shift838]

On some orders (not all) I will get the below critical error when I go to the status/comments tab of the order to put in comments.  I will only get this error if I toggle the 'Notify Customer' button to 'ON'.  Many of my orders will work fine, but on some I get the below when doing the exact same thing that works on others.

Code Select Expand

There has been a critical error processing your request
Error: Could not load model checkout/fast_checkout
Trace:
#0 /home/shiftcom/domains/shift838.com/public_html/store/core/helper/utils.php:1608
#1 /home/shiftcom/domains/shift838.com/public_html/store/admin/model/sale/order.php:1143
#2 /home/shiftcom/domains/shift838.com/public_html/store/admin/controller/pages/sale/order.php:1156
#3 /home/shiftcom/domains/shift838.com/public_html/store/core/engine/dispatcher.php:306
#4 /home/shiftcom/domains/shift838.com/public_html/store/core/engine/dispatcher.php:341
#5 /home/shiftcom/domains/shift838.com/public_html/store/core/engine/page.php:110
#6 /home/shiftcom/domains/shift838.com/public_html/store/core/engine/router.php:204
#7 /home/shiftcom/domains/shift838.com/public_html/store/core/engine/router.php:86
#8 /home/shiftcom/domains/shift838.com/public_html/store/index.php:97
in /home/shiftcom/domains/shift838.com/public_html/store/core/engine/loader.php on line 126