When we went to Custer, and were standing at the host site, he told us we had to make a reservation AND pay a reservation fee even tho we were standing right there in the park at an empty site. We drove up to the only section that is first come, and no hookups, and then we still had to call and "reserve the site", and pay the reservation fee.
Custer also requires everyone to buy the day-pass, even if you camp. Custer is a great park, but has a buncha add-on fees, so be prepared for it. If you're camping any other areas in the state, just buy the monthly day-pass and it covers all parks. As I recall it, the day pass was $6 and a month was $15, but am not sure. You might want to open the park website and check current fees.
This was two years ago - not sure if it's still the same ... Custer gets pretty full, so I'd get the ducks in a row ahead of time if you're traveling a distance. You pay the reservation fee regardless - at least when we were there.
