I have been told that the NPS somehow monitors the names of the boats in the anchorages to see if they have paid. If not, the charter company gets dinged for the fee.
On a recent charter with IYC, I was asked to save the stubs from the envelopes and give them to IYC at the end of the charter.
This is true. I don't think it is every boat every day, but as Dirty Harry said, "Do you feel lucky?"