Product Purchase, Run CGI

Hello all,
I'm new to Zen Cart. I've got it setup look and feel wise pretty much how I'd like it.
Now, I'm selling software that I write. The customers will use my main site to download 30-day trial versions, and then need to purchase a key through the Zen Cart store to activate it for further usage.
How would I go about, after the checkout is complete (I use PayPal only) to provide them with a key. To generate the key, I must run a CGI (C++) program which can return the result in any way necessary. Do I need to modify the checkout PHP to look for specific product names hardcoded and then execute the command or can it be an option name/value.

I would hook into the order class in the loop that works its way through all the attributes and add the additional text with the generated key directly to one of the attribute descriptions.

• check to see if matches hard-coded product ID number, and hard-coded option name/value pair, call external program to generate key, save the generated key as part of the attribute option-value text in the order.

Likely somewhere between:
//------ bof: insert customer-chosen options to order--------
and
//------eof: insert customer-chosen options ----

Sounds good. I was just wondering if it was able to be done via some options, or been done already.

Thanks.