XRechnung and ZUGFeRD via API, that recipients accept.

import { generateInvoice } from '@normapi/client'

const { xml, rulesetVersion } = await generateInvoice({
  invoiceNumber: 'RE-2026-0815',
  issueDate: '2026-08-13',
  currency: 'EUR',
  buyerReference: '04011000-12345-03',
  seller: { name: 'Muster Software GmbH', vatId: 'DE123456789' /* … */ },
  buyer: { name: 'Beispiel Handel AG' /* … */ },
  lines: [
    { name: 'Software licence', quantity: 3, unitPrice: '199.00', vatRate: 19 },
  ],
}, { apiKey: process.env.NORMAPI_API_KEY })

curl -X POST https://api.normapi.de/v1/invoices \
  -H 'Content-Type: application/json' \
  -H "Authorization: Bearer $NORMAPI_API_KEY" \
  --data-binary @invoice.json \
  -o invoice.xml

# As a ZUGFeRD PDF: ?syntax=zugferd and -o invoice.pdf

<?php
$ch = curl_init('https://api.normapi.de/v1/invoices');
curl_setopt_array($ch, [
    CURLOPT_POST           => true,
    CURLOPT_POSTFIELDS     => file_get_contents('invoice.json'),
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER     => [
        'Content-Type: application/json',
        'Authorization: Bearer ' . getenv('NORMAPI_API_KEY'),
    ],
]);

$xml = curl_exec($ch);
if (curl_getinfo($ch, CURLINFO_RESPONSE_CODE) === 200) {
    file_put_contents('invoice.xml', $xml);
}
curl_close($ch);