Hmm... weird. I think you did everything correctly.
Try combining both methods in the file:
mntroot rw || exit 1
test -e /mnt/us/RUNME.done && echo "Not running, RUNME.done exists" && exit 0
echo 0 > /var/local/upstart/lab126_gui.restarts
test -f /usr/sbin/shipping_mode || echo "shipping_mode does not exist"
test -f /usr/sbin/shipping_mode && /usr/sbin/shipping_mode
Also, just to be on the safe side: make sure to save the file with Unix line endings (not windows ones).