Where the parties have agreed to a delinquency charge in accordance with s. 422.203, Stats., and instalments have subsequently been deferred, the merchant may collect a delinquency charge on any deferred installment which is not paid in full on or before the 10th day after its deferred due date unless such installment is again deferred.
Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.271