Host charged me for a service I did not accept

The 08/09/2014 at 1:16pm we(customer/guest) decided to book Sharon’s (host/vendor) place (https://www.airbnb.com/rooms/294657 cf copy of the ) through the mobile phone for the same night.
At 3:45pm we still hadn’t any news of the host and believed that she wouldn’t answer us as the check-in was supposed to be at 4pm (in 15min). We tried to cancel the booking request but couldn’t find any button on the mobile app.
She finally answered at 3:55pm saying that the place would not be ready before 7pm and asked for our approval for the booking since the conditions had changed (cf first mail of Sharon). Thus, as the contract conditions had changed the host or Airbnb should wait for the customer approval before charging the expense as specified by the FTC “charges for service you didn’t accept or that weren’t delivered as agreed”. (check in supposedly at 4pm being finally at 7pm)
We did not answer to Sharon’s mail and de facto did not approve the new contract (check-in at 7pm instead of 4pm). Meanwhile, at 4:47pm (47min after the normal check in), Sharon took the responsibility to accept the reservation, without our consent on the new conditions (check-in at 7pm instead of 4pm), which resulted on charging us the total amount of the night: $401.
As a matter of fact, I am requesting that the error be corrected, that any finance and other charges related to the disputed amount be credited as well, and that I receive a total refund of the $401.