Welcome to cruise-chat.
See what happens when you don't use your agent?
All I can say is you are stuck, given you are already within the cancellation period. You can play phone tag with NCL and only get more frustrated. In the big scheme of things, it is only $200 so I wouldn't let that ruin my cruise. I just wouldn't forget it when the time came to book my next cruise.
Chalk it up to "lessons learned". I'd think an agent could get the onboard credit too and perhaps would have ensured it was $300 before finalizing things.