For support on the RetroFlag switch, you should ask them on their support/project page.
I'd still recommend the script by @cyperghost, it's easy to use and has really detailed instruction both in the topic started and on the github page.

I figured out the issue. Problem was I got an error when I followed MSL's instructions to the letter. First time I got an error saying something like "this can not be a service", second time it said "Failed to execute operation: Bad Message". I mistyped. The script:

[Unit]
Description=RemotePi Board Shutdown Service
Before=multi-user.target
Conflicts=shutdown.target
[Service]
ExecStart=/bin/true
ExecStop=/etc/shutdown.sh
Type=oneshot
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target

I had "Service" on a line by itself. Doh! It's fixed now, powering on off works perfectly now. Hooray!

@mediamogul

I ended up making a little pull tab...

Thanks, I'm going to have to give this a try!