If the post office didn't comply with/lead an investigation into who mailed it (or something like a mail bomb - there are ways to do direct damage with the platform as well, imagine the site being used to host viruses) and instead said that they didn't moderate what went through the mail, they will protect free speech, mailing is a form of speech, and that they won't enforce anything for the spirit of the platform, the platform would be eviscerated and the we must protect the children backlash would be enough to get a law passed saying that every single bit of mail must be opened or read. For the internet platform, they'd have to start having questions about what they enforce and what they don't, and when they do or don't work with law enforcement. Just look at how Apple is going up against the justice department and multiply that conflict by 100x