'Your grace' would be for a duchess. 'My lady' or 'Your ladyship' would be permissible. Or simply, Baroness.

Though no one here seems to care of such things. But your land really has no nobility?

[Tazendra knew what a republic was, since the Dragaeran Empire occasionally became one for religious regions. But noble titles persisted through it. A government with a complete lack of nobility was alien to her.]