Don't know anything about an extra charge for Windows, but not all laptops come with Windows installed, especially in some areas. You need to look at the laptop that was purchased and see what it came with.
These are the Far Cry 4 minimum requirements, which means semi-smooth play at low settings
http/www.systemrequirementslab.com/cyri/requirements/far-cry-4/12380/?p=r
The GTX 460 card they recommend got a 2639 rating in PassMark 3D test, the 940M got a 907, 950M got 1410. If you want to buy a slower laptop for the game, up to you to take the chance of it playing poorly and at low settings. I don't recommend anything under a 960M card for a "gaming" laptop, even for a lower end gaming laptop, which would be about $800-900 in US.