The popup notifications in Xubuntu have always annoyed me, they pop up right over top of the window controls and generally just get in the way. It is the xfce4-notifyd service that is responsible. The popup notifications are almost never informative, but some apps depend on the service, so I can't just uninstall it. So I set about finding out how to disable the notifications without actually removing it. It turns out it's fairly simple:
sudo nano org.xfce.xfce4-notifyd.Notifications.service
Change it to look like this:
Now the popups will stop but those apps depending on it (like the volume controls) will still work.