E‑invoicing API: a guide to compliance in Germany

The clock is rapidly counting down to the date when all German businesses will be obliged to bill their B2B customers via e-invoicing. In the race to achieve compliance, software companies need to quickly get up to speed on the most appropriate solution to offer their clients.

Upcoming e-invoicing compliance dates in Germany

• From 1 January 2027 all German businesses with a turnover exceeding €800,000.
• From 1 January 2028 all businesses in Germany.

One major aspect of e-invoicing is how EN 16931compliant e-invoices are transmitted. In Germany, businesses can choose between sending their structured XML documents via email or making use of the Peppol e-delivery network.

An e-invoicing API comes into play when opting for transmission via Peppol. Before explaining what an e-invoicing API is, it’s good to first understand why a German business would choose Peppol over email to send e-invoices – and thereby need an e-invoicing API.

Email vs Peppol for e-invoicing

There are pros and cons to each transmission choice.

Email pros and cons

Using email does not require any new infrastructure or workflows to be put in place.

However, sending e-invoices via email involves a large manual workload for downloading, validating and importing documents. As well as creating an additional time burden, this leaves a lot of room for error. And that’s not all. Email transmission is notoriously insecure, with no traceability, and is unsuitable for large volumes. Interoperability with recipients’ systems is also not guaranteed.

Peppol pros and cons

Peppol is a structured, interoperable network designed specifically for secure, automated exchange of e-invoices and other procurement documents. E-invoices flow directly between systems, complete with automated technical compliance validation. Minimal human intervention means minimal workloads and errors. Peppol also has a strong audit trail and is future-proof due to its alignment with EU-wide digital VAT initiatives (ViDA) and cross-border interoperability.

As with any new system implementation, adopting Peppol does require infrastructure and workflow change, for which sufficient time needs to be allowed.

Side by side comparison

While email is the minimum viable option for e-invoicing transmission, Peppol is the strategic route for the long-term. It is also by far the most advantageous for medium and large businesses with significant invoice volumes, and the best option for companies operating across borders or with public sector clients.

E-invoicing APIs explained

As the full name – application programming interface – suggests, an API is a set of rules that enable different applications to communicate with each other.

An e-invoicing API specifically supports the exchange of invoices between ERP/CRM (issuing) and accounting (receiving) software via an e-delivery network, such as Peppol, without having to work via an external party or platform.

E-invoicing APIs are specifically designed to ensure safety and security around invoicing transactions by protecting all the relevant data from unauthorised access, tampering, or fraud.

However, e-invoicing APIs may vary in quality, depending on the Access Point Provider they are built and maintained by. The safest e-invoicing APIs incorporate robust authentication, verification and other security measures, including SSL/TLS encryption, OAuth for authorization, and are subject to regular security audits.

As well as carefully scrutinising the Peppol Access Point Provider behind an e-invoicing API, German businesses need to ensure they get an e-invoicing API that complies with the standards and regulations set out in Germany’s e-invoicing mandate.

The Flowin e-invoicing API

Flowin is an e-invoicing API built specifically to connect with Peppol. Compliant with all German requirements and offering the highest levels of security, Flowin enables fast integration with ERP and CRM systems while offering a uniquely advantageous cost model.