<cfset _PAYLOAD = serializeJSON(fulfillment_json)>

<cfoutput>#_PAYLOAD</cfoutput>

Payload: {"fulfillment":{"message":"Your package was shipped.","location_id":"30406673","tracking_info":{"number":"9405511206207180455478","url":"www.usps.com"},"notify_customer":false,"line_items_by_fulfillment_order":[{"fulfillment_order_id":5784060395699,"fulfillment_order_line_items":["{\"id\":12254030430387,\"quantity\":1},{\"id\":12254030463155,\"quantity\":1}"]}]}}

I'm pretty sure it's just escaping your quotes. https://stackoverflow.com/questions/15637429/how-to-escape-double-quotes-in-json