All versions of TurboTax charge a fee except for TurboTax Free, which is free for anyone, regardless of income level. But if you upgrade from Free to +Plus, Standard or any other version, then charges apply (again, regardless of income level).
Unfortunately, if you started your return in a product other than TurboTax Free, or you upgraded to another version, there is no way to go back to Free. You would have to go to the Return Manager (under Tools) and start your return over in Free.