add a secret code on the invoice after payment

[Godfried]

Hello,

I looked on the forum, but can't find a topic about my question.
Joomla 2.5.9, Virtuemart 2.0.18a, jumi 3.0.3-2

I would like to add a secret code on the invoice after payment.
It is a secret code which comes with a product, a cutomer buys, to be used as an password to do an online course.

My idea was to add a custum field (editor) to a product.
In this editor I can put some "jumi php code" to create the secret code.

Only thing to do is just to display the custom field after someone has payed. Not before!

I can't find the file to change the code to achieve this.

Maybe someone has a better idea.

Thanks for your answer!