As part of AirBnB's Houston's flood assistance program, I've been hosting a family. Their reservation is meant to expire tomorrow and they wish to stay till next Friday. As they are currently without a home, I would like to extend them at no cost, but the AirBnB booking system is not allowing me to extend their stay without charge. When I set the cost to zero in the alteration page, the system tells me that the cost of a reservation may not be less than $10. How do I make this change without charging my guest?