I have been reading the forums here and I noticed that many people who have custom number plates from the Ifruit app in singleplayer find their plates don't appear online.
I have figured out a way to get your custom plate for online.
All you need to do is the following:
1. Go onto the Ifruit app and into LSC
2. Click onto your online character (their name is actually your social club username)
3. Go to the plates tab and, change the design/colour from what you have now to something different. Example if you had the black plate click on the Blue plate etc.
4. Click on the order button and you should get an invoice pop up confirming the plate for your online character.
5. When you boot online up next time you'll get a text saying your plate is ready to be picked up. NOTE THIS MAY NOT OCCUR STRAIGHT AWAY. It took 2 real life days for me to get the text saying the plate was in. SO DON'T FREAK OUT
And that's it. Also note that you can't change what your plate says between online and offline in-case you are curious.
Hope this helps!