change line 1064:
$WA_redirectURL = "checkout_success.php";
to:
$WA_redirectURL = "";
then change line 1117 - 1120
//Send Mail All Entries
if (""!="") {
header("Location: ");
}
to:
//Send Mail All Entries
if ("checkout_success.php"!="") {
header("Location: checkout_success.php");
}