It is possible to prevent Item Fulfillments from being sent to Shiptheory when creating them in NetSuite. In order to do this, ensure that the "Sent to Shiptheory" custom field is checked when creating the Item Fulfillment. This custom field is created when the Shiptheory SuiteApp is installed on your NetSuite and can be found in the "Custom" section of your Item Fulfillment creation forms. 

Important note: If you uncheck the check box and re-save the Item Fulfillment and the Item Fulfillment matches the trigger status required to send the shipment to Shiptheory, the shipment will be sent to Shiptheory. Therefore it is of the utmost importance that this field is left alone if you wish for the Item Fulfillment to continue to be blocked from sending to Shiptheory.

If you wish to verify that the cause of this was not a glitch in our system then navigate to Customization > Scripting > Scripts in NetSuite. Search for the script file "shiptheory_webhook.js" and view it. 

Go to the execution log section and you should see a log stating that the shipment you just saved has already been sent to shiptheory. The number in the message corresponds to the internal ID of the item fulfillment you just saved. 

To find this value on your Item Fulfillment view the Item Fulfillment and check the URL for the query parameter "id=<Item Fulfillment ID>".