How an Activation Key automatically gets emailed to the user after they purchase/download the software?

Yes, such a solution is already included with the Activation Server. It is called the KeyGen CGI. The KeyGen CGI provides an interface between the Activation Server and the third party ecommerce providers. As soon as the payment is authorized, the provider can call the KeyGen CGI to issue the Acitvation Key on-demand. The issued Activation Key as well as the user information will be saved in the database. The Activation Key will also be returned to the provider to be emailed to the user automatically. The KeyGen CGI is currently compatible with Share-it, one of the leading e-business solution providers. The full source code of the KeyGen CGI is provided. So you can customize it to interface with any ecommerce providers as well.
Below is a demo link for the KeyGen CGI.
Assume your payment gateway server is done with charging the user's credit card, it would call the KeyGen CGI just like the example link above. When you hit the submit button, you will see an Activation Key is returned. Your server would email this Activation Key to the user. You can also check out the Activation Server to see that the issued Activation Key and user account are saved in the database automatically.
The demo site is available at:

Username: Administrator

Password: Administrator