If you confirmed a trip and didn’t show up for the position, then you are a 'no-show'.
This is a very difficult situation for the host, mainly because the host is counting on receiving the volunteers to help him out at his property. If the volunteer doesn’t turn up, he's going to be under-staffed and unable to run his business properly.
For that not to happen, all you need to do is cancel the trip if you are not going to it! Pretty simple and quick, and it avoids letting the host down and taking him by surprise.
This gives him a little more time to plan how to substitute the volunteer & is the right thing to do.