There are many situations where Id like a tick box in pricing.
In my tent space, extra if you use my tent
In my tent space: extra if you borrow an air mattress.
In any room, extra if you bring a pet
In any room, extra if you want a cot. I charge a flat rate for one or two guests, extra for the third but twice this week guests wanted a cot made up for just two guests. I can't charge after the fact?
My guests love instant book, special offer is a drag. All we need is a blank "Extra" or two. Host fills in extra description and price, guests ticks it during instant book.