Offer XRechnung for invoices

[jowlo]

As a german government agency, i want to be able to download my invoice in the XRechnung XML standard, because that is the mandatory format for public mandates in my state.

The XRechnung standard is the german implementation of EN 16931-1 already mandatory in some states and for federal agencies. We should offer the option of downloading an XRechnung XML file for customers.

The Koordinierungsstelle für IT-Standards has a FAQ (german only) including some technical aspects. They also provide tools at https://github.com/itplr-kosit that can be used to verify compliance.

Acceptance criteria

• [ ] For business customers, the button for downloading invoices in Payment Settings will show a drop down that offers either "PDF" or "XRechnung"
• [ ] If XRechnung is selected, then the invoice will be downloaded in an XRechnung-compliant XML Document
• [ ] If PDF is selected, then the normal PDF is downloaded

Tasks

• [x] research/define the which schema we will use to generate XRechnung (seperate XML file with CII syntax)
• [x] specify which fields to use in the XML document (probably the same as in the PDF invoice
• [ ] create a class which generates XRechnung-compliant XML documents (template based is probably simplest)
• [ ] rename the PdfInvoiceService to DownloadInvoiceService and extend it to allow specifying a return format of either PDF or XML
• [ ] adapt the client to use the DownloadInvoiceService so that business customers can get XRechnung invoices

some links

https://github.com/itplr-kosit/xrechnung-testsuite https://github.com/itplr-kosit/validator-configuration-xrechnung

[johnbotris]

Should we bump priority on this, since it is a requirement?

[jowlo]

I am not sure how hard the requirement is, i think as of now it is only for government agencies as customers themselves, but it might be transitive. I think basically it makes it easier for other businesses to import the invoice. @armhub should know a bit more.