Interest for any legal indebtedness must be at the rate of six percent per annum unless a different rate not to exceed the rate specified in section 47-14-09 is contracted for in writing. Unless otherwise agreed by the parties in writing, all contracts must bear the same rate of interest after maturity as they bear before maturity. A charge for a late payment penalty may be imposed only if the amount of the late charge or the method of calculation of the late charge has been agreed to by the parties in the loan documents that are signed by the borrower.