Add Shortcode Automatically to Each Post

I added the short-code here to give you an example of how it should look:
http://networktokyo.com/_wp/?p=5130#more-5130

Basically I want to add a reply form on every post, inserting the email address of the (non-subscribed) author of each post.

Just like a classified website I want to have a simple form placed just under each Posted Content for the reader to reply. That reply is sent to the author’s email address they used to post the ad.

If it’s possible this would be great to ad specifically for certain categories. For example if I wanted to post news, I don’t want a reply form on that category.

So first I need to add a field called “Email” on the “Post A Free Ad” form. But before I do that, is there any way I can automatically insert a Short-code, (connecting Contact 7 form for example) to the post content, while replacing the email on the form to the email that the author uses when they posted their ad?

I am also open to adding contact form code as I did with the “Post A Free Ad” page.

Or perhaps it’s easier to replace the comment form with a email reply form?

I am very new to php… I’m a VB guy, but am willing to learn php which is why I chose WordPress as my web projects this year.

Thanks in advance.

dw