Maybe the ranger station doesn’t prefer to receive mail directly. The NPS page for Isle Royale National Park lists the mailing address for the park headquarters on the mainland (in Houghton, MI 49931) in the box where most parks would list a physical address.
For what it’s worth, locals on this forum maintained in 2011 that mail should be addressed to Grand Portage, MN 55605, in care of the ferry.
Interestingly, the person who raised it with us said zip 55606!
It is substantially more complex than that, ranging from countries with proper postcode areas to small scale delivery point schemes. Even countries that in general have actual postcode areas tend to have individual codes for special cases.
PS leaving out the discussion that in countries with modern postal systems postcode areas are largely defunct and at most only used as an additional tiebreaker in the case of ambiguous addresses.