If the rental of any safe-deposit box is not paid within six months from the day it is due, the bank, or at any time thereafter and while such rental remains unpaid, may mail a notice to the person in whose name such box stands on its records stating that if the amount due for such rental is not paid on or before a specified day, which must be at least 30 days after the date of mailing such notice, the bank will cause such box to be forced open.