Well, that is quite a big question and has a few parts.
But you are in the right place. order.php is where the work will get done.
Firstly, have you created the password already? If not then that is a whole different subject.
You need to check that that particular product has been ordered. You'll do that in the 'for' statement that loops through all the products. It starts with this line at about 696
for ($i=0, $n=sizeof($this->products); $i<$n; $i++) {
In there you will want to text is the current product is the right one . You'll use something like:
if( 100 == $this->products[$i]['id'] )
{
// what you want to do goes in here.
}
That would test if the product is id=100 and then do something. I am writing the code off the top of my head so there may be typos in there :-) You can put that pretty much anywhere in the 'for' loop but see what I say lower down
Now for what you actually want to do. Well that is kind of up to you. But you can either send a separate email or add the information to the existing email.
To send a separate email look at the function zen_mail and have a look at how it is used on line 1050ish of order.php.
To add the information to the existing email you'd add the string to the $custom_information. There are some comments about how to do that at about line 880 of order.php.
Hope that all helps. It is hard to comment further without more information.
