Yes. Follow the instructions posted near the bathroom facility. Fill up the envelope, insert the money, tear the stub off the envelope. Drop envelope in the box. Lastly post the stub using the clip on the site # pole. Once you claim your site you can go off to explore. Have fun!

Yes there are envelopes there to fill out and leave part at the site to claim.

Each campground is different, but you should be able to leave your site once you have paid, and your reservation time begins. We highly recommend leaving chairs, a traffic cone, or water jug marked with Reserved on it in the entrance to the site to indicate you are there are returning. You might also ask a neighbor to keep an eye out for “squatters”

Yes, if there's a spot available when you get there, then you can claim it (I don't remember exactly, but I assume you'd pay and post it on the site) and then leave for the day to explore and come back.