I'm not sure if they do. The oxidation of silver can be reversed, but it requires the plaque to be separated from the bag so that it can be treated to reverse the oxidation. There is no need to polish it, which can thin it out and lighten up the engravings.
I went to the Fendi nearest to me and asked about it. They recommend replacing the whole metal tag at a few hundred dollars oh well, not that it's a big deal. I shall try polishing it first and see if it works. Else if it doesn't, I can just replace the entire tag