Wis. Stat. § 72.045: Timely filing.

Documents and payments required or permitted by this chapter that are filed by mail are on time if they are mailed in a properly addressed envelope, if the postage is paid, if the envelope is postmarked before midnight of the due date and if the department or the person that the department designates receives them no later than 5 days after the due date. Documents and payments that are not mailed are timely if they are received on or before the due date by the department or at the destination that the department prescribes.