Wis. Stat. § 891.18: Affidavits of service.

Whenever any notice or other writing is by law authorized or required to be served the affidavit of the person serving it, setting forth the facts necessary to show that it was duly served, shall be presumptive proof that such notice or writing was duly served. But this section shall not apply to any service where another way of proving such service is expressly prescribed by law.