Wis. Stat. § 195.13: Rebates and concessions, unlawful to accept.

It shall be unlawful for any person, firm or corporation knowingly to accept or receive any rebate, concession or discrimination in respect to water carrier transportation of property wholly within this state, or for any service in connection therewith, whereby any such property shall, by any device whatsoever, be transported at a less rate than that named in the tariffs in force, or whereby any service or advantage is received other than is therein specified. Any person, firm or corporation violating the provisions of this section shall forfeit not less than $100 nor more than $1,000 for each offense.