E-invoicing API for software vendors

Your software sends invoice data as JSON and gets back a validated XRechnung or ZUGFeRD PDF – archived and delivered on request.

Why now

If your software writes invoices – ERP, shop, industry solution – your customers need e-invoices from 2027 and 2028 (§ 27(38) UStG).

What the API takes care of

• Calculation according to EN 16931: totals, taxes and rounding, so the business rules are met.
• Formats: XRechnung in UBL and CII, ZUGFeRD in the EN 16931 profile; for Romania also RO e-Factura.
• Validation: every generated file with the KoSIT validator and the official rules (XRechnung, CIUS-RO for Romania); incoming invoices can be validated and read as well.
• Archive: every issued invoice, unalterable and with a checksum.
• Delivery: by e-mail; delivery over Peppol is in preparation.
• Events: signed webhooks when an invoice is issued or delivered.

How to start

1. Create an account.
2. Create a test key in the portal and try everything in the sandbox – test invoices are not counted.
3. Follow the quickstart: one invoice with a single call.

The API reference describes every endpoint; the sources of the SDKs for TypeScript, PHP, Java and C# are ready, publication in the package registries follows.

Why an API instead of your own implementation

The rules of XRechnung and EN 16931 change with new versions; KoSIT publishes validator configurations for them. With an API these changes stay with us, and your software keeps a stable contract (the API is versioned in its path, see versioning).

Frequently asked questions

How much does the API cost?

The plans are on the pricing page: cancel monthly, billed by issued invoices and active sellers. Validation is unlimited in every plan.

Where is the data processed?

Server and database are in Frankfurt am Main; e-mails are sent through MailerSend in the EU. Details are in the privacy policy.

Can I issue invoices for several of my customers' companies?

Yes. One account can create several sellers; billing follows the active sellers in the month.

Sources

• XRechnung (KoSIT) and ZUGFeRD (FeRD), German
• § 27(38) UStG on the 2027 and 2028 deadlines (German)

As of 19 October 2026. This guide is not tax or legal advice.