Ok, well then that is the solution... Usually the success page is specified as part of the payment processing.
If you have universal email, just update the redirect in that server behavior. Otherwise you will have to update the code where the email is sent to add the redirect page manually...
You would update it from:
if (""!="") {
header("Location: ");
}
to:
if ("checkout_success.php"!="") {
header("Location: checkout_success.php");
}