The USPS does investigate content.

You can draw a swastika and a machinegun for sale on your regular mail envelope and it will show up in informed delivery, but as black and white.

If you try to get it displayed more prominently in an advertising campaign, it violates their second set of 'guidelines' that stop what you can put in the more prominent colored advertising image.

They use this mechanism in a matter different than most social media curation, but it's still a form of curation, and favoring the particular kinds of speech they like, using two different sets of guidelines -- one guideline for de minimis B&W presentation and a second set of 'guidelines' (which even at USPS are a bit vague) about whether you can get the pretty color image in informed delivery.