@splaquet
I posted an issue on woocommerce github and got replied by their staff, said the problem has been transfer to ShipStation extension team.
https://github.com/woocommerce/woocommerce/issues/21676
Same problem.
Fatal error: Class ‘WC_Integration’ not found in /home/coolin6/public_html/myheatsinks.com/store/wp-content/plugins/woocommerce-shipstation-integration/includes/class-wc-shipstation-integration.php on line 10
