В файле paymentmodule.php найти:

Код:
if (intval(Configuration::get('PS_INVOICE')) AND Validate::isLoadedObject($orderStatus) AND $orderStatus->invoice AND $order->invoice_number)
                    {
                        $fileAttachment['content'] = PDF::invoice($order, 'S');
                        $fileAttachment['name'] = Configuration::get('PS_INVOICE_PREFIX', intval($order->id_lang)).sprintf('d', $order->invoice_number).'.pdf';
                        $fileAttachment['mime'] = 'application/pdf';
                    }
                    else
                        $fileAttachment = NULL;



и закоментить вот так

Код:
//if (intval(Configuration::get('PS_INVOICE')) AND Validate::isLoadedObject($orderStatus) AND $orderStatus->invoice AND $order->invoice_number)
                    //{
                        $fileAttachment['content'] = PDF::invoice($order, 'S');
                        $fileAttachment['name'] = Configuration::get('PS_INVOICE_PREFIX', intval($order->id_lang)).sprintf('d', $order->invoice_number).'.pdf';
                        $fileAttachment['mime'] = 'application/pdf';
                    //}
                    //else
                    //    $fileAttachment = NULL;


Судя по зарубежным сообщением, способ работает, но у некоторых как и у меня возникает ошибка Invalid order or invalid order state при завершении оплаты.