Pin delivery after successfull Checkout

Hi guys,
I have been learning to modify open-cart to my needs. I have been successful to add information what i need without hacking into the core and partially using vqmod. Now i am confused with one thing. i would really appreciate your help and guidance. basically I want to achieve the following :

After success full checkout --> for each bought product (i.e calling card A) a Pin will be shown to the buyer. now I would prefer a solution with which i minimize the problem during opencart version update.

now i am confused which files to modify or where should i place my code. since its a kind-of "Instant Delivery" I want to make sure, its only delivered securely and after payment.

Thanks in advance for the help.

cheers
ps: i am using OPEN CART v1.5.1.3