Wis. Stat. § 75.002: Timely payment.

(1) When payment is required under this chapter to be made on or before a certain date, the payment is timely if it is mailed in a properly addressed envelope, postmarked before midnight of the last date prescribed for making the payment, with postage prepaid, and is received by the proper official not more than 5 days after the prescribed date for making the payment.

(2) A payment which fails to satisfy the requirements of sub. (1) solely because of a delay or administrative error of the U.S. postal service shall be considered to be timely made.