I don't remember any self-pay option for hiker/biker sites, at least in the state parks I stayed at (Oregon/California only). In each case I checked in at the booth entering the park. Especially in Oregon, they were very strict about wanting to make sure those staying in hiker/biker sites were really touring hiker/bikers, presumably because of the problems California has had with transients moving into the sites. I wanted to stay a second night at Ft. Stevens near Astoria, and the rangers actually had to get approval from a manager before they would allow me to do that. Having said that, I think I'd bring some checks anyway. They don't weigh much. Every campground I've ever stayed at out west that did have a self-pay box has accepted checks.

As to the stakes, each hiker/biker site I was in was on solid ground where any kind of stakes would have been fine. No sandy campsites. However, there are lots of state parks and I didn't stay in all of them.