The plugin is available from here: WordPress PHP-KeyCodes
It is similar to the stand alone application PHP-KeyCodes which is described on https://www.withinweb.com/phpkeycodes/.
If you are a software developer, then using license keys is a way to ensure that your products are safe and secure. You use this application to automatically distribute the codes after payment from PayPal.
The pin numbers are listed in the database one entry per line. When a purchase is made, PayPal sends an IPN notification to the plugin which then extracts the first pin number, sends it to the purchaser and then removes that pin number from the list.
The email sent to the purchaser contains the pin number or key code, and you will receive a copy of the email.
The sales history listing also identifies which pin number has been sold to the purchaser.
A local test system is included which allows you to test without connecting to PayPal.
The system has a high level of security in that the license key code will not be sent out until the correct PayPal return code is received with the correct purchase values. The system checks that the database value is the same as the amount that has been spent, and the currency code is the correct value. This ensures that any alteration to the button code will not work.
The installation into WordPress is the same as for any plugin as is the procedure for upgrades.