I think I'd argue that the Postal Reorganization Act of 1970 is to blame[0]. In short, postal workers prior to this law were not allowed to engage in collective bargaining. They went on strike to protest this, and this bill, in response, permitted mail employees to organize. In exchange, however, the US Post Office Department, which was formerly directed a cabinet position, was abolished and replaced with a corporation-like independent agency (the USPS as we now know it).

The USPS is expected to produce almost all of its own funds and has had most of its pre-1970 subsidies cut back or eliminated. At the same time, the prices it charges are still set by the federal government (specifically by the Postal Regulatory Commission, appointed by the President), meaning it can't truly set rates that allow it to be profitable[1]. The effect of all this essentially means it's forced to try and generate revenue elsewhere, like through these ads.

[0]: https://en.wikipedia.org/wiki/Postal_Reorganization_Act [1]: https://en.wikipedia.org/wiki/United_States_Postal_Service#R...