When complaint has been made to the commission concerning a rate for a product or commodity furnished or service performed by a public utility, and the commission has found, after investigation, that the public utility has charged an unreasonable, excessive, or discriminatory amount therefor in violation of any of the provisions of this part, the commission may order that the public utility make due reparation to the complainant

therefor, with interest from the date of collection if no discrimination will result from that reparation. No order for the payment of reparation upon the ground of unreasonableness shall be made by the commission when the rate in question has, by formal finding, been declared by the commission to be reasonable, and no assignment of a reparation claim shall be recognized by the commission except assignments by operation of law as in cases of death, lack of legal capacity to make decisions, bankruptcy, receivership, or order of court.