When you create an FBA shipment directly on Amazon for an item you also fulfill locally, you need to decrease the item's local inventory in Monsoon Marketplace before you add it to a shipment. In this example, we have 20 units of an item in local inventory. Of those 20 units, we need to add 10 units to an FBA shipment on Amazon.
On the My Inventory tab, select the item that you will to add to an FBA shipment.
Click the Quantity ellipsis (...) button.
Change local quantity from 20 to 10 (deduct the amount you will add to an FBA shipment), then click OK.