Create rule to email unique registration code

I'm not sure if I really got your idea. Could you please elaborate on what you really want to achieve? Perhaps a step by step description of what the user does and how the system should react.

I think you have to code a custom-module for that. You could do that like this: hook into your e-commerce module and generate a new registration key as soon as an order is paid. Then send a mail to the customer.

In the case of drupal-commerce that would probably look like this (disclaimer: I never did that, the code is incomplete and untested):

<?php
/**
 * Implements hook_commerce_payment_order_paid_in_full().
 */
function myregcodeintegrationmodule_commerce_payment_order_paid_in_full($transaction) {
  // regcode_generate($length, $output, $case);
  $regcode = regcode_generate(8, 'alpha', false);
  $code = (object) array(
    'code' => $regcode,
  );
  // regcode_save($code, $terms = array(), $action = REGCODE_MODE_REPLACE);
  regcode_save($code);

  $mailaddress = somehow_figure_out_email_address_via_transaction($transaction);
  drupal_mail(..., $mailadress, ..., $regcode, ...);
}
?>

+1 for this feature request (and thanks for the code).

Actually, let's use #1507710: Define a Rules Action for creating Registration Codes as the issue, since that is specifically about creating the missing feature of a RulesAction to generate a registration code.