Letterboxing is allowed BUT it requires a Special Activity Permit which costs $30 every three months to maintain. Source of information was an e-mail from Crowder's Mountain State Park (Gastonia, NC) Superintendent, Larry Hyde on 12/14/06 after pulling letterboxes from the park that did not have permits.

NC State Park Special Activity Permit

Documented policy is no longer located on any state park website but can still be viewed here.