Need to change quantity on shipment

I created a shipping plan to send some stuff over to FBA only to later realise that that I don’t have as many units as I have stated in the plan. As it is already submitted ( marked as shipped, but the goods have not been shipped yet) I cannot change the quantity my self.

So, I contacted SS and they said they have changed the status to ‘Working’ - but status but changed to “ready to ship”. which means I still can’t amend any of the information. When I mentioned this to SS, they said 'Yeah, that’s the status" and when I pressed, they said 'Wait 24 hours" and try again and it should change and closed that chat. I am not really convinced it will change after 24 hours because the intial status change to ‘Ready to Ship’ happened in seconds.

Guys, have you had this experience / should I wait this long or just go ahead and ship the goods with the incorect quantity?