Hi Ashley and thank you for your reply.
That is not good enough, quickbooks should introduce some more options as there's quite few companies paying this way. I've set up the payment terms +45 days, but this doesn't add END OF THE MONTH into the equation which makes all payment dates inaccurate.
I assume I could try to set it up to worst possible scenario ea 75 days (45+30) and see how this will work.
Best Regards
Simon