From January 1, 2004, the requirements of invoices presented in Germany have changed. This came up in the comments to an earlier entry that mentioned an article on the BDÜ website. These requirements apply particularly if you want to set off an invoice against VAT.
First query: what form of electronic signature will be accepted? Does it require extra hardware or would one of the two or three forms of signature in Adobe Acrobat suffice?
It looks to me as if you do need the hardware, so I will go back to sending invoices on paper and wait for more advice. Here is some information in German).
Another requirement is that invoices are numbered consecutively. Information for German lawyers in two PDF files (here and here) at the Bundesrechtsanwaltskammer site is partly relevant to translators. (via Vertretbar.de) There is a transitional period until 30 June 2004 in which this rule need not be followed. (Does this period apply to the electronic signature too?)