amazon seller account and FBA

So I saw two plans for amazon seller acc, one is individual and other is professional but when trying to select either, it gives me a notice that it will charge 39.99$ usd every month, but I see on other places it only take 0.99$ per unit I sell, is there something I don’t know or it will take 39.99$ both on sign up and monthly? And can anyone tell me how FBA works, a good tutorial or in your own words.