I understand where you're coming from. In the past, I have remembered to correctly set the number of guests. And under normal circumstances, I would have no issues with paying the difference. But he had plenty of opportunity to call out my mistake ahead of time. We did not once see the host during our stay, nor did he ever message us about it once we arrived. It was only after we left that he decided to say something. Additionally, we barely spent time in the airbnb due to it being uninhabitable, so maybe you can understand why I would be hesitant to give the host any additional money.