I have not heard from Artio for these 20 days. Support is thus not existing, do not put your hopes up to get a response. Besides this entry on the forum I mailed them twice, all no reaction. really sucks.
I used VM Invoice 2.0.14. Then I thought, let's try the newer version, altough I could not find a list what was changed, so it was a real long shot. Deinstalled the 2.0.14 version, installed the 2.0.19 version, all settings where kept. So I upgraded to version 2.0.19 and this software had even new items for this discount problem that I had.
So the problem is solved.
I can now really automatically send these invoices.