Option not to run post script if app or update fails

We regularly use postscripts and noticed they are still running when app installs or updates fail. We only want the postscript running if the app install or update completes successfully. We regularly update information in the uninstall key in our post scripts if the vendor doesn't by default (i.e. - installdate, source, etc.) as we want that inventory and now it is writing this info to the registry even when it fails which we obviously do not want.