The normal course of appeal would be to escalate through Nikon Service.
If you don't have any luck there, try escalating through your retailer to the local or regional Nikon rep.
Don't stop appealing the repair cost until you have gotten a positive action or have made your way to a senior level person.
If the shutter was bent, possible sources of the problem are a previous purchaser at your retailer, your retailer, or Nikon Service when examining or removing the damaged shutter. The shutter is not a part you would normally touch - and it sure makes sense that an effort to "fix" a stuck shutter or to remove it could cause damage.