Sounds like they will stand their ground on the return and I can see their point since it was mounted, even if it was their mistake. Say to yourself that they screwed you but you will not let it tear you up and get you bent out of shape.
Buy the Nismo studs and spacers from another vendor and stop buying from Z1 from now on. Don't start up a website labeled Z1MotorsportsSucks though I got a laugh out of that last hate website (can't remember which product it was). Move on. Lesson learned.
Though I relearned from your experience today so feel good about that.