Corporate monopoly on national park fees

The insufficient competition in the US allows businesses with market power to gouge by piling on "junk fees". Booz Alan has a monopoly over some kinds of access to some US national parks and other public lands, exercised via a "government" web site run by that company, and pulls in much more income from junk fees than the US government agency concerned actually gets.

But it's worse than that. Aside from the matter of price, use of that "government" web site requires running nonfree software. Those parks are off-limits to the free world.

I have visited some US national parks, and I paid cash to enter them. Is that still possible? Can anyone investigate that site (see the article) and report which parks and places can't be entered by paying cash, without using any web site?

Or which parks and places can still be entered by paying cash, without any web site?

Whichever list is shorter would be the more useful.