Using the g-brief  document class one can write professional fancy-looking business letters according to the appropriate DIN standard.
Especially useful are the commands for printing folding marks and window marks, if you intend you send your letter in a long DIN evelope with a transparent window.
Since the Deutsche Post AG sells stamps as QR code pictures in PDF format one can easily extract the stamp from the PDF and include it in the window using the picins package ,. Unfortunately, other methods fail to place the QR code correctly in the address field.
To get started with a document one needs quite a lot of commands. See my example TeX file for more details.