@Alma0, because your settings indicate 'extra people No Charge', it's possible the guest thought the price included 1-2 persons.
(Unfortunately, this has been a matter of confusion in the past.)
It's necessary to state in the 'extra guests' box the amount you expect to receive from the second person.
Another option is to get rid of the 'extra people' box altogether. You can do this by going to your settings, entering a very large amount, and then hitting 'Save'. When you open the page anew, the box should be gone. (This was a tip I got from an experienced host here.
You could then raise the price slightly, and indicate in your listing that it is for 1-2 people. Or mention in at least two places in your listing that there is a further charge for a second guest. Another option is to disable IB, and when someone requests to book you can ask specifically how many guests will be arriving.
In your shoes, I would not pursue the matter further, since guests in principle should not be expected to adhere to conditions not indicated clearly in the listing. In any case, the matter is not covered by AirBnB protection, as it's the host's responsibility to be clear about the price.
When you checked the guest's itinerary, was it not indicated there that the number of guests would be 2?
I'd also suggest not using Instant Book, especially until you have fine-tuned your listing regarding specifics.
I'd recommend charging a Security Deposit, and making your House Rules more specific and comprehensive.
Not everyone treats their own home with respect, and so may - as strange as it sounds - be ignorant of what a respected home might look like in our view.
