in the checkout button code, remove this chunk so that it will not append the query string:
if (isset($_SERVER['QUERY_STRING']) && $_SERVER['QUERY_STRING'] != "") {
if (strpos($Redirect_redirStr, "?") === false) {
$Redirect_redirStr .= "?".$_SERVER['QUERY_STRING'];
}
else {
$Redirect_redirStr .= "&".$_SERVER['QUERY_STRING'];
}
}