Full payment is required at the time of booking (years ago, it used to be different, but now it's all due immediately).
At 5:00PM, they close all the interactive areas (the river, lagoon, rays, reef, dolphins, and aviary). However, the gift shop and photo sales area stay open for around a half hour after that while people buy all their stuff (I recommend doing all this purchasing /before/ 5PM, however, as the lines get long.). The bathroom / shower areas will also remain open until about 5:30PM as well.
This is the same all year round for the basic package.
If you opt for the summer "Twilight Discovery" package, that's a different deal... you do not enter the park until around 6PM and then stay until around 9PM.
=)