The reason this is a problem is because the user is making use of the back button in the browser. The checkout page is the page that sends you off to PayPal, but depending on weather or not you clicked on the PayPal button on the cart this page will either display for your or send you to PayPal.
To resolve this issue you can just add in the following code directly after line 15 on the checkout.php page:
php:
<?php if(WA_Auth_RulePasses("Using Express Checkout Only")){ // Begin Show Region
header("Location: cart.php");
}
?>
This will ensure that the user is taken back to the cart page that they came from rather than the checkout page.
