FIOS requires you to have some sort of device to subscribe to TV.
Either a STB or cable card.
Although cable cards are cheaper, I'm not sure if they will eventually detect that you have not activated it.
They won't let you.
Any time you try and return the box, when they check your account and see that is your only TV device, they will not process a return.
You can give it a try to see if maybe things have changed, but doubtful.