It won't let me put on my custom plate online....
- Turn off console
- Order it in the app for your fourth vehicle (new one appears for "online")
- Wait a while, go have a coffee or a quick masty or something
- Come back, start console, once in it should be on that car and you should be able to attach it to any other for $600 at LSC.
If those exact steps do not work for you, then f**k, because that's what I worked out I had to do when I did it (doing it other ways like ordering while in-game never worked for me).