Assuming that these PINs emails will be sent manually, simply create your products as products; when purchased, the client receives an order acknowledgment. You can then go to the orders page on admin, clcik edit and then send a "dispatched" email and add your comments; your comments include the PIN.
It may be worth making it obvious to your client that they will receive 2 emails; you could even word it so that they have to respond to the confirmation in order to get the PIN - an extra layer of security.
On the other hand, if you are intending to generate the email with the PIN automatically, then I'm afraid I can't come up with any suggestions at the moment - apart from perhaps using the order id no as the PIN.