I seen that they approved cancellation very close to shipment. That supriced me.
But now then have produced the case for you so I'm not sure that's doable when it's so close to send off from factory. Maybe you get lucky.
If they can change the delivery address for a European order that is yet to be shipped from the distribution centre (which they have according to someone on the Slack), they can cancel a slacker backer order that is yet to leave the factory.