When toner sales men ship you a crate of toner cartridges you didn't ask for, and then sends you a bill for those toner cartridges, you don't have to send them back, and you don't have to pay for them either. Welcome to the real world.
http://www.snopes.com/crime/fraud/supplies.aspThat's only because the law explicitly allows for this, as a way to prevent unscrupulous scammers from sending you things on purpose just to compel you to buy them. It's a notable exception to prevent abuse of the mail for fraud.
If toner salesmen park a truck full of toner cartridges in your driveway, you don't suddenly own their truck and their toner, as the exception no longer applies.