Wis. Stat. § 98.08: Price refunds; price information.

(1) A person who uses an electronic scanner to record the price of a commodity or thing and who sells the commodity or thing at a price higher than the posted or advertised price of that commodity or thing at least shall refund to a person who purchases the commodity or thing the difference between the posted or advertised price of the commodity or thing and the price charged at the time of sale.

(2) A person who sells a commodity or thing and who uses an electronic scanner to record the price of that commodity or thing shall display, in a conspicuous manner, a sign stating the requirements of sub. (1).