You want a USPS form 1500 (https://about.usps.com/forms/ps1500.pdf https://about.usps.com/forms/ps1500.pdf). The intended use is for reporting obscene mail but

You want a USPS form 1500 (https://about.usps.com/forms/ps1500.pdf https://about.usps.com/forms/ps1500.pdf). The intended use is for reporting obscene mail but the decision about whether something is objectionable is left up to the addressee (Rowan v. United States Post Office Department - https://www.law.cornell.edu/supremecourt/text/397/728 https://www.law.cornell.edu/supremecourt/text/397/728).