First of all: Thanks a lot to the developers of IP, great work!!!
I created a small Invoice-PDF-Template based on the one shipped with IP 1.5 originally. Its accordingly to the DIN 5008 Form B standard and can be placed perfectly in common german envelopes with a window. It also includes marks for folding.
DIN-5008-konforme Rechnungs-PDF-Vorlage für Sichtbreifumschläge mit Falt-/Lochmarken
Feel free to use and customize the template: https://github.com/tirun/invoiceplane_din_5008_b