Oracle SQL Summing Values From Two Different Tables
You can do this using an UPDATE:

UPDATE inventory i
   SET quantity = ( SELECT s.quantity+i.quantity 
                     FROM shipment s
                    WHERE s.itemname = i.itemname );


or a MERGE:

MERGE INTO inventory
USING 
(
  SELECT itemname, quantity
  FROM shipment
) ship ON (ship.itemname = inventory.itemname)
WHEN MATCHED THEN UPDATE 
    SET inventory.quantity = ship.quantity + inventory.quantity;


Example SQL Fiddle here if you want to play.