OK, I see the problem, the results page is set to filter the recordset on the ProductName for, element from the serch page, but when the add to cart button posts, that form element does not exist any more which causes the initial search results to be lost.
add the following code at line 1 to store the search term in a session and reset the post when the add to cart button posts:
<?php
@session_start();
if(isset($_POST['ProductName']) && $_POST['ProductName'] != "") $_SESSION['ProductName'] = $_POST['ProductName'];
if((!isset($_POST['ProductName'])) && isset($_SESSION['ProductName']) && $_SESSION['ProductName'] != "") $_POST['ProductName'] = $_SESSION['ProductName'];
?>