If you have already confirmed your Amazon shipment plan, quantities can only be changed by small amounts.
If you have already approved a shipment plan, Amazon FBA will only allow the quantity of each Amazon SKU to be edited up to 5%, or up to 6 units, above or below the original amount without incurring fees.
If you need to add more units, you will need to duplicate the shipment or create a new shipment.
You cannot add additional Amazon SKUs.
For more information see Amazon’s Help Center.