Redirection and URL in Browser...

Hello ! I'm adding products to my shopping-cart with a php-script like add.php?product. IMPORTANT: It's NOT possible to use SUBMIT or any Java-Script-Stuff. And I don't want to use META-Redirection, becaus it shows up in (Netscaps) Browser-History, which makes it impossible for the user to use the "Back"-Button. The problem: When the user presses "reload" on the browser, the product is added again, because the script add.php?product is called again. Any idea how to handle this ? I tried to do the following: When pressing "buy" on a produkt, the script add.php?product is called, but it's making no output, instead I use Header("Location: show.php") to redirect to shop.php, which shows the cart. The problem: This works, but in the adress-bar, the browser still shows add.php?product. If the user presses "Reload", the product is added again... Any ideas ? Regards Martin