When booking a cabin is the entire fee due up front or do you pay the balance when you arrive?

You pay upfront as the rangers office isn't always open .

We were only visiting for a day, don't know about that. I would call the Park services and ask them.

Pay upfront. You may arrive when office is closed. But have no fear they have a system to get you the key. Tip: If you are arriving in winter, get the approved list of firewood vendors. Last time we stayed, our guy stacked it out of the weather and gave us some firestarters!