We paid for six nights in one envelope. It has a place to write the senior park pass number and the envelope has a tear off strip with a control number to show you've paid. No one checked on us any of the nights we were there. Also, outside the National Park Service building there's a box where you can get a paper slip they call a "coupon" that lets you pay with credit card. We found this easier than cash. The charge went through about two weeks after we deposited the envelope in the floating pay station.