Yes, many electric cars have a buffer. Not all Tesla though. That's why it's not recommended to charge a Tesla to 100% everyday while it's fine with many other brands.

But Tesla did sell a few cars with different battery capacities and exactly the same hardware. If you got the smaller capacity then you got a buffer. Tesla could unlock the buffer remotely, if you give them more money or if you need to evacuate before an incoming hurricane or similar.

Yeah I think fair to charge the customer more to access more of the total battery capacity because Tesla is now assuming more warranty risk on the battery.