How can I charge for pitching extra tents on my camp site?

I host a small camp site where I charge per guest including the pitching of their tent. Sometimes the size of the group booking means they bring extra tents with them understandably so I need to charge for the extra pitches. Would best way to do this be listing the charge per tent in the description and then go through the Resolution centre? I can't see any other way to do it.

Thanks.