Navigate to the Settings page and click “Selling Channels” 

locate your Amazon store you want to change and click on the ellipses, then “Edit” 

on the Popup page, under the field “Deducting inventory from FBA shipments”, click on the Dropdown to select whether the inventory should be deducted when the shipment is in “Working” status or when the shipment is in “Shipped” status. 

Please note, on the item activities, the FBA shipment will always be shown on the date of the shipment creation. The above-mentioned setting is only applied as to when the inventory should actually be deducted.