Change the Flexible to "Flexible: must contact host xxx days beforehand to confirm"
If you choose flexible Airbnb translates it as "anytime after check in start time " Anytime is a long time in a day to be waiting around especially when guests assume that can can turn up anytime after ... without notice. I know I can choose and end time but that doesn't work for me or guests.
btw: A lockbox with key does not work for me, I have to be here to let them in.
I always ask people for arrival time, but some do not get back to me or call me 20 minutes before they arrive and expect me to magically appear on the doorstep to meet them. I am flexible if given notice but I cannot sit around all day and night on the off-chance. Reason for suggesting this is that I do not appreciate being penalized by Airbnb or dinged by the guest because they arrive with no prior warning and then complain to Airbnb that I am not around or they had to wait for 10 minutes and my listing says "flexible", even though I state in my listing that they must tell me what their e.t.a is. I want to be flexible but not subject to a guests carelessness or whim or a penalty from Airbnb.