Adding an include in a form.. Can I do it?

Ok,
Here's the deal. I have a web form that is going to serve as a registration
form for events my company will be sponsoring. Some of the events will be free,
some will not. For the pay events, I need to additional information, like
payment details, etc. I am hoping that I can pass a URL Parameter to the
registration form like "?charge=yes" to call an include that will extend the
basic form into a larger one with the payment fields on it.

I have everything done, however, when I pass the "yes" parameter, the
additional fields show, but they are on top of my basic form fields. Can
someone point me to where I have gone wrong?

Thanks,
Chris

here is an example

<form name="form" method="post" action="something.php">
<!-- begin the normal form elements -->
<input type="text" name="textfield">
<!-- end the normal form elements -->

<?

// check if the url variable charge is set and the value is yes

if( (isset( $_GET['charge'] ) ) && ( $_GET['charge'] == yes ) {

// if so include the following file
include('filewithotherformelements.php');
}

?>
</form>
etc...

Thanks teiz, gonna try it and see what shakes out.