$_REQUEST / $_SESSION

Hope someone can help with this, as I'm from a design rather than coding background.

Basically I've just about finished off integrating an ecommerce site with Protx, whereby some customer data is collected on the main site (contact info mainly) before being passed to protx's payment gateway.

In the first instance, the Protx script collects various fields, by way of $_REQUEST - eg :

$ThisVendorTxCode = $_REQUEST['VendorTxCode'];
$ThisAmount = $_REQUEST['Amount'];
$ThisCurrency = $_REQUEST['Currency'];
$ThisDescription = $_REQUEST['Description'];
$ThisCustomerEmail = $_REQUEST['CustomerEmaill'];

etc.

However, the customer is required to log in with an email address before ordering, so their email address is held in a session.

So what I thought was all I needed to do was replace another form text field 'CustomerEmail' in the form, and

$ThisDescription = $_REQUEST['CustomerEmail'];

on the following confirmation page, with just

$ThisCustomerEmail = $_SESSION['Email'];

on the confirmation page, and that from then on, it would be fed through just as the $_REQUEST['CustomerEmail']; would.

But it seems not.

Am I barking up the right tree here, or is there some fundamental reason why this isn't working?

Hope that makes sense!

Cheers.