It is understood and agreed that should this application be approved, payment for all purchases during a given month is due and payable on or before the 10th of the following month. After the 20th of the month, any unpaid balance will be subject to an interest rate of 18% per annum (1 1/2% per month) If full payment is not received by the end of the month following purchases, the account will be closed until payment is received.