It's fairly easy to opt out of unsolicited ("junk") mail in the US [1]. I'm not sure what happens if you try to send bulk postcards to people who opted-out. I think either the post office doesn't deliver them (but you still pay) or you open yourself up to fines if people complain.
Postal Agent might want to auto exclude people who have opted-out on the DMAchoice list [2] if it doesn't already do it.
This service will not eliminate all promotional mail. It does not apply to promotional mail from companies or organizations that consumers have an existing business relationship. It also does not apply to local retailers, restaurants, and political organizations. You should expect to see a major reduction in the overall volume of prospect promotional offers received. Please note that ANA is not the source of consumer names for marketing purposes. This opt-in mail suppression service is a tool to assist consumers in managing their marketing offers.
The fact of the matter is postal marketing is return positive from a marketing perspective, and it will continue so long as marketers spend, consumers respond, and mail delivery agencies need the revenue.
in the US, this type of junkmail pays for the postal service to keep it in the black, so that non-profitable one-off mail of personal interest can be delivered quickly and affordably.
Also some national postal services sell this "feature".