Open the product in your panel and add codes, one per line. They are stored encrypted, and each one is handed to exactly one order.
When a buyer's payment is confirmed, the system takes a code from the pool and puts it on their order immediately. You do not have to be awake.
If the pool is empty, nothing breaks — the line simply waits for you to deliver it by hand, and you will see it in your orders list.
Automating delivery is the single biggest thing you can do for your listing: no waiting for the buyer, no late-cancellation window, and fewer disputes.