Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.07
A delinquency or default charge is not a finance charge within the meaning of s. 421.301(20), Stats., if imposed for actual unanticipated late payment, delinquency, default or other such occurrence. However, when a merchant's billings are not paid in full within a stipulated time period and under such circumstances the merchant does not, in fact, regard such accounts in default (For example, by customarily failing to institute collection activity or by continuing to extend credit) and imposes charges periodically for delaying payment of such accounts from time to time until paid, the charge so imposed comes within the definition of a finance charge and the credit so extended comes within the definition of open-end credit.
Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.07