Do I have to pay for charging?
NO! Charging is free at all Tesla Supercharger locations (identified on the cars GPS map), so your only cost will be the rental fee and then whatever toll charges you incur - no charge for gas like you would with a regular car, so the Model S is perfect for lengthy road trips.
How does auto-pilot work?
Auto-pilot is an incredible feature, especially for rush hour driving. You need to be alert when using it, and always ready to take over - but it's really life changing functionality.
First, Auto-pilot is a terrible name for this system :) if you don't pay attention for any extended period of time, you will crash - 100%. I can't tell you how many times I've had to move the car back into my lane, grab the wheel to avoid a median, etc. But it's also really amazing, and can be an awesome tool. It mostly works very well.
There are a few different pieces:
a) Adaptive cruise control. You use the shorter stalk on the lefthand side of the wheel (below where the wipers and turn signal is), and pull it towards you once to set the adaptive cruise speed. Then you turn the end of the stalk towards you or away from you to change the number of car lengths between you and the car in front of you (I suggest using at least 3, and probably 4 or more on the highway to be safe). The car will go the max speed you set it at until it senses a car in front of you, and then it will slow down and keep the distance between you and that car that you've designated (based on number of car lengths). It is fantastic. One thing to be aware of is that if the cars in front of you slam on their brakes, you do not want to rely on autopilot to stop for you. It probably will, but I have heard reports of people rear ending cars in front of them because it didn't stop soon enough.
b) Full Auto-pilot: So when you start driving, you'll see a tiny outline of a steering wheel appear to the right of the speed you're traveling on the dash. When that little wheel is visible, it means autopilot can see the lines and is available. You'll also be able to see images of the other cars on the road, and the lines the car is able to see - they'll be grey initially, and once autopilot is engaged, they will turn blue. When the little steering wheel is visible, you pull the same stalk you used for adaptive cruise back towards you twice, and you should hear a beep, and see that the little steering wheel and the lines will turn blue. It works on highways and also on regular roads - and it's great for stop and go traffic. When you're close to another car in stop and go traffic, or when the car loses the lines and autopilot is engaged (and it can see the car in front of you) you'll see that the autopilot will target the car in front of you instead of the lines, and that car will turn blue. The Tesla will then follow that car's path instead of the lines - and it will often switch back and forth.
Okay those are the basics - I've also attached a link to some YouTube videos below.
There's also some functionality called Summon where you can move the car remotely from the key FOB - here's a link to how that works.
Here's a video on perpendicular auto-parking (parallel parking works similarly, though will not work on an end space... has to be parking in a spot with a car on each side):
And there are a bunch of others, as well. If you have any questions or issues, you can probably find what you need on youtube or in the Tesla forums, or don't hesitate to call.
Have a great day w the car, and take good care of the car!