I know/hope this is a joke but people used to do this at an independent, third party, mail sorting place I worked at as a teenager.

They’d mail all sorts of shit (both literal and figurative) and it was basically handled by one guy who seemed ok with it but was definitely not ok.

Mail them lead, weights, whatever. But please, no matter how despicable the company, there’s likely some at very least mildly abused worker who is just trying to earn a wage and has to deal with the vile shit people try to punish companies with.