The $6 is charged per permit. If you book several nights at one time you are charged the $6 plus your nightly camping fee. If you book each site individuality you are charged $6 each time because the system views it as mailing several permits.
Kids 16 and under are always free, if you have 5 over the age of 17 and 20 under 17 your fee is $20 per camp per night. There is a cap of 25 bodies (including dogs) per site, some sites are smaller though.
Your group does need to launch together and stay together on the river. This is a stipulation on the permit and if you have a group that launches the next day, you could potentially get a ticket for breaking a permit stipulation.
If there are any more questions, I would be happy to clarify!
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