Cancels will happen, its a part of the business. It sucks, but it happens on both ends. The right thing to do is at least contact the individual and let them know you are going to cancel. However, they may be an instance where this doesn't happen. I know the last thing I want to do if i just got a phone call that a relative died is call a lady to say I won't be there. Not saying I wouldn't make that phone call, but i think it really depends on the relative.
WIth posting alerts I would at the very least give a few days, and contact the person to see what is going on. If you get some bullshit post an alert, or share the information. If its legit, give them a chance to make it up to you or simply just never see them again.