Using Session Variable & Recordset to show Discount

I'm struggling with sending an order copy to the client after returning from PayPal.

On the Order Confirm page generated by the Checkout Wizard I have a UE behaviour to advise the vendor of the order.

Discount is set by matching a form field value to a value in the database with a corresponding discount. This works perfectly on the viewable page as well as the email.
As this page loads, I am also creating a session variable which = the value entered in the coupon field.

Parameter on recordset is:

<%
Dim coupons__Param
coupons__Param = "-1"
If (cStr(Request.Form("coupon")) <> "") Then
coupons__Param = cStr(Request.Form("coupon"))
End If
%>

Customer then checks out with PayPal and returns to the site to a thankyou page which again shows their order and triggers an email to them.

Because the Form is now history, the value used above is no longer available so I edited my Recordset on the Thankyou page to use the Session variable:

<%
Dim coupons__Param
coupons__Param = "-1"
If (Session("coupon") <> "") Then
coupons__Param = Session("coupon")
End If
%>

This does not work on the displayed page or the email generated by UE. The discount does not show. All other variables show up properly.

I realize this covers more than UE, but I believe the solution is one and the same. If I can get it on page, it will be in the email, so how do I retrieve this value that was entered on the Checkout page and populate the Discount section of the Shopping Cart.