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...
https://www.mymove.com/about-us/
https://www.washingtontimes.com/news/2011/oct/12/postal-serv...
https://money.cnn.com/magazines/fortune/fortune_archive/1992...