eInvoice Tools

Tools that work together

Read and check e-invoices. Understand validation reports.

Here you will find our tools for e-invoices, that is, invoices as structured files that software can read. At the center is the free eInvoice Viewer. It shows you any e-invoice in readable form, and every other tool connects to it: the validators for the official KoSIT Validator, for Mustang and for VAMPIRE check the invoice on display at the click of a button, the Validation Explorers explain every finding right at the field of the invoice, and eight paid add-ons extend the viewer with further tasks, each in a tab of its own. That way you open, check, understand and process an invoice in one single place.

All tools run as WordPress plugins on your own website. The viewer reads the file in the browser; the validators and Validation Explorers process it on your own server. None of the tools sends the invoice to a third-party service or stores it. The viewer, the three validators, the three Validation Explorers and six of the add-ons are also available as an app without WordPress, for your computer, your intranet and SharePoint. Which tool comes in which form is shown in the section “As an app, as a WordPress plugin or in Outlook” further down.

	eInvoice Viewer	Validators (KoSIT, Mustang, VAMPIRE)	Validation Explorers (KoSIT, Mustang, VAMPIRE)
Display the invoice in readable form	✓	–	–
Recalculate totals and point out gaps	✓	–	–
Check the invoice against the official rules	–	✓	–
Explain each message at the invoice field	–	–	✓
How you get it	free	free	paid products

What the three basic tools do for you and how you get them.

eInvoice Viewer

The viewer is the central tool for your e-invoices. It shows them on your WordPress site or in the app as easy-to-read documents and reads ZUGFeRD and Factur-X, which are PDF files with an embedded XML invoice, as well as plain XML files in the CII and UBL forms, including XRechnung. Since version 2.38 it organizes everything in tabs: “Invoice” shows the EN 16931 fields, “XML” shows the file line by line with a search, and “PDF” shows the pages of the PDF invoice. Every validator and every add-on with a result gets its own tab next to them. That way you stay with one invoice and only switch the view. The viewer is free.

Go to the eInvoice Viewer

Validation Explorers

A validation program usually reports errors only as rule codes and technical paths in the XML. The Validation Explorers turn this into understandable text: for every message they show the affected EN 16931 field, an explanation and the actual value from the invoice. Together with the viewer, every finding appears right at the field of the invoice, so you know at once what to change. There is one explorer each for the official KoSIT Validator, for Mustang and for VAMPIRE. All three are paid products and require the free viewer.

Go to the Validation Explorer for KoSIT

eInvoice Validators

Three free plugins connect your WordPress site to a validation engine: the Validator for KoSIT to the official KoSIT Validator, the Validator for Mustang to the Mustang validation program and the Validator for VAMPIRE to VAMPIRE PHP. Each gives your site a validation page and the viewer its own check button. The KoSIT Validator and Mustang need Java on your server, VAMPIRE does not. With the paid VAMPIRE JS add-on, the page of the Validator for VAMPIRE even checks the invoice in your visitor’s browser without sending it to your site.

Go to the validators

The eight viewer add-ons

The viewer is free. Each of these eight add-ons is a paid product of its own and adds one task to it. In WordPress you install it as a second plugin next to the viewer. Six of the add-ons also come as an add-on package for the viewer app, which you simply place in the app’s folder. From viewer 2.38 on, every add-on with a result appears in a tab of its own next to the invoice, and you do not have to open the file a second time.

Viewer – Exporter

Writes every field of an invoice you have opened to an Excel file, with field number, name and value. That way you can keep working with the data in your spreadsheet. As a plugin next to the viewer.

Go to the add-on

Viewer – Extended Profiles

Also shows the additional fields of ZUGFeRD and Factur-X EXTENDED in their place in the invoice, such as further parties, logistics charges and cash discount terms. As a plugin and as an add-on package for the viewer app.

Go to the add-on

Viewer – Technical Details

Shows the data type, cardinality, XML path and related business rules for every field and opens the field, rule and reference browsers. As a plugin next to the viewer.

Go to the add-on

Viewer – Visualizer

Shows the invoice as a printable document in five layouts in the “Layout” tab. In the plugin it outputs a PDF/A-3 with the original XML embedded; in the app you print it from your browser. As a plugin and as an add-on package for the viewer app.

Go to the add-on

Viewer – Identifier Check

Checks the identifiers in the invoice, such as VAT ID, GLN, LEI and IBAN, for structure and check digits and marks wrong ones right at the field. In the plugin it can also ask the responsible register. As a plugin and as an add-on package for the viewer app.

Go to the add-on

Viewer – Inbound Check

Compares incoming invoices with your company data and your supplier list and reports, for example, an unknown bank account for a known supplier. As a plugin and as an add-on package for the viewer app.

Go to the add-on

Viewer – EPC QR Code

Shows the EPC QR code of the invoice in the “Transfer code” tab, known in Germany as GiroCode. Your banking app reads the beneficiary, IBAN, amount and remittance information from it, so nobody has to type in the transfer. As a plugin and as an add-on package for the viewer app.

Go to the add-on

Viewer – Hybrid Check

For ZUGFeRD and Factur-X it compares the PDF you see with the invoice data in the XML: invoice number, total amount and bank account. That way you notice when the XML names a different IBAN than the PDF, a well-known pattern of invoice fraud. As a plugin and as an add-on package for the viewer app.

Go to the add-on

As an app, as a WordPress plugin or in Outlook

Most of our e-invoicing tools are not only available as WordPress plugins but also as apps, and the Inbound tool (eInvoice Inbound) also comes as an add-in for Outlook. For us, an app is a folder that you download as a ZIP file and unpack. You open its start file with a double-click in your browser, with nothing to install and no server to run. The app reads the invoice in your browser and does not upload it anywhere. Your IT team can also put the same folder on the intranet, show it in SharePoint or embed it in your own website.

The app and the plugin of a tool are one product: the same features, the same display, the same name and the same version number. You simply choose the packaging that suits the way you work. The table shows which tool comes in which form and when you can get it.

Which tools are available as an app, as a WordPress plugin and in other forms, whether they are free or paid, and when you can get them
	App	WordPress plugin	Other form	Free or paid	Availability
eInvoice Viewer	yes, one folder for your computer, intranet and SharePoint	yes, on wordpress.org	demo on invoicePress.de	free	app available for download today, plugin on wordpress.org today
Viewer add-ons Extended Profiles, Visualizer, Inbound Check, EPC QR Code and Hybrid Check	as an add-on package for the viewer app; there the Visualizer prints through the browser instead of creating a PDF file	yes, as a second plugin next to the viewer	–	paid	plugins and add-on packages in the shop
Viewer add-on Identifier Check	as an add-on package for the viewer app; there it checks structure and check digits on your computer, the register lookup is available only in the plugin	yes, as a second plugin next to the viewer	–	paid	plugin and add-on package in the shop
Viewer add-ons Exporter and Technical Details	no	yes, as a second plugin next to the viewer	–	paid	plugins in the shop
Validation Explorer for KoSIT	yes	yes, requires the viewer	–	paid	in the shop
Validation Explorer for Mustang	yes	yes, requires the viewer	–	paid	in the shop
Validation Explorer for VAMPIRE	yes	yes, requires the viewer	–	paid	in the shop
Inbound tool (eInvoice Inbound)	yes	yes	add-in for Outlook	app and Outlook add-in free, plugin paid; add-ons VAMPIRE JS and Supplier List paid	app available for download soon, Outlook add-in coming soon to AppSource, plugin coming soon to the shop
Rule Studio for VAMPIRE	yes, with VAMPIRE JS included	–	–	paid	coming soon to the shop
eInvoice Validator for KoSIT, for Mustang and for VAMPIRE	no, because validation runs on your server	yes	–	free	planned for wordpress.org; the Validator for VAMPIRE has been submitted there
VAMPIRE JS for the Validator for VAMPIRE	–	yes, as a second plugin next to the Validator for VAMPIRE; the validation page then checks in the visitor’s browser	–	paid	in the shop
VAMPIRE PHP	–	included in the Validator for VAMPIRE	library for developers, via Composer from Packagist, source code on GitHub	free, GPL-2.0-or-later	on Packagist and GitHub today
VAMPIRE JS	as an add-on package for the viewer, Inbound and Explorer apps, also in the Outlook add-in	–	library for Node 18 or later and for the browser	paid	coming soon to the shop

The app is the better choice if …

you do not run WordPress or do not want to install anything,
confidential invoices should not leave your computer or your company,
the tool should appear on your intranet, in SharePoint or in an existing website outside WordPress,
one person works with it, for example in accounting or at a tax advisor’s office.

The plugin is the better choice if …

you already run a WordPress site,
several users with roles and permissions should work with it,
visitors or customers should open or check invoices directly on your website,
you need uploads, storage and a connection to a validation engine such as the KoSIT Validator or VAMPIRE.

The Outlook add-in fits if …

your invoices arrive by email in Outlook with a Microsoft mailbox, especially with Microsoft 365. The Inbound tool then appears as a pane next to the open email and reads the attachment directly, without you having to save it first.

In short: the app is a workplace for one person. If you need permissions for a team, use WordPress. The quickest way to get to know the app form is the free viewer.

Download the viewer app for free

VAMPIRE: validate e-invoices without Java, in PHP and in JavaScript

Several tools on this page are powered by VAMPIRE, our own validation engine. It checks XRechnung, ZUGFeRD, Factur-X and Peppol BIS against the official validation rules, but unlike the KoSIT Validator and Mustang it does not need Java. We measured it: for 220 out of 220 test invoices, VAMPIRE reached the same result as the official KoSIT Validator, as of October 9, 2026.

VAMPIRE PHP

Runs on PHP 8.2 or later on ordinary web hosting, without Java. VAMPIRE PHP is free software under the GPL-2.0-or-later. You install it with Composer from Packagist, and the source code is on GitHub. In WordPress, the free Validator for VAMPIRE brings it along.

VAMPIRE JS

Runs in Node from version 18 and in the browser, even in an HTML file that you open on your computer with a double-click, without any web server. As an add-on package it brings full validation to the apps and to the Outlook add-in without uploading the invoice. For the WordPress plugin eInvoice Validator for VAMPIRE, VAMPIRE JS is available as an add-on of its own: the validation page on your site then checks the invoice in the visitor’s browser, with the same report as on the server, and the invoice is not sent to your site. VAMPIRE JS is a paid product.

Get to know VAMPIRE

Try it in the playground

WordPress plugin Validator for VAMPIRE