The stock is sent by you or your manufacturer physically and for that purpose you have to create shipments and send stock and need to enter tracking ids so it is nearly impossible to automate this process. Howver, recently Amazon provided a service called Amazon distribution Centre, you can send your stock in large quantities to Amazon distribution centre and Amazon will automatically replenish your inventory at fulfillment centre, as long as your inventory is available at Distribution centre, you will not go out of stock.
