An individual who receives an overpayment or an improper payment of a benefit from the retirement system may be liable for the amount of the overpayment or improper payment, plus interest, accruing at a rate of five percent per annum beginning on the 30th day after written notice by ERS of the overpayment or improper payment. ERS may, in its discretion, decline to charge interest under this rule if the overpayment was due to an error by ERS, or if the cost of calculating, assessing and collecting the interest may reasonably exceed the value of the interest.