Well, that's easy:
You entered 2 guests as maximum no. of guests you take.
And the second person will have to pay an extra fee of 18,-
If I booked alone, I'd pay your basic amount.
If I booked for me and my partner, there'd be an extra 18,- on top of the basic amount for extra usage etc.
You can of course decide to choose for $ 0,-.
However, the system will then state Extra guests: No charge.
Silly and misleading (and yes guests interpret that to mean the more the merrier),
but that's how the system is now. So better if you have an extra fee per person after the first guest.