That sounds pretty suspicious to me. Writing you letters trying to squeeze money out of you is not what I would expect Walmart to do. It sounds like a scam artist to me, not Walmart. You should call the company headquarters directly (not the number on the letters they are sending you) to verify they are coming from Walmart.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.