Tools around electronic invoice.
To me it looks like they all use the same java library, eventually Mustang?

Link collection

Quba
https://quba-viewer.org/
https://github.com/ZUGFeRD/quba-viewer

Mustang
https://www.mustangproject.org/einvoices/?pk_campaign=quba&pk_source=hp

Elster (all pointing to same tool)
https://www.elster.de/eportal/e-rechnung
http://www.erechnung.elster.de
http://www.e-rechnung.elster.de

24port (online site but does not upload invoice)
https://24port.eu/erechnung/

Electronic invoice