If you cannot find the serial number, or you have deleted the email, you can find your code in your Order History by going to Origin > EA Account and Billing... > Order History or directly by clicking My Account.
Primarily this plugin will enable you to sell and manage license keys. When a customer purchases a license enabled product, a license key will be added with the order when the order is complete. You can also set orders to auto-complete for your license enabled products. License keys are displayed on the order confirmation page and also added with the order confirmation email. Your customers will be able to find the serial numbers in their account section under the order details too.