public function getInvoiceNrSourceOptions()
    {
        return array(
            PluginConfig::InvoiceNrSource_ShopInvoice => $this->t('option_invoiceNrSource_1'),
            PluginConfig::InvoiceNrSource_ShopOrder => $this->t('option_invoiceNrSource_2'),
            PluginConfig::InvoiceNrSource_Acumulus => $this->t('option_invoiceNrSource_3'),
        );
    }