There isn't a special way of handling 30/60/90 terms and the like. We'll move that further up the to-do list for later updates.
For now, you might add a note in the Description area of the payable and you can see this when you cut checks because you can widen the Cut Checks screen to see more columns of information.
This would be in addition to setting the due date - most like for the first of the due dates.
