Reserve America runs the Oregon State Parks reservation system.
There is an $8 fee to make any reservation. There is an $8 fee to change any reservation. Full payment for the entire camping period is made at the time of the reservation.
And then this
An example: You have a reservation for eight nights, but decide to drop three nights. The nightly rate is $25. You will be charged $75 (3 times $25) plus an $8 transaction fee for a total of $83.
Yes, to cancel if made more than 3 days before - they keep the $8 reservation fee, and change fees, and the first night's camping costs. (It appears to be cheaper to cancel the eight night reservation in the example above quoted from the Oregon web site - and book a new reservation for five nights only.)
Texas runs their own in-house reservations systems - one of the few that doesn't use Reserve America as the front end (or backend like recreation.gov).
Texas Parks & Wildlife is looking into ways to limit/ eliminate the busy weekends sale of reserved spots. They know it is a problem.