If you remove the need for external funding, you also remove the political baggage that comes along with it. Cost centers in businesses are bad to be in because there is always someone around trying to squeeze you instead of support you.

And unlike police who, when trying to be profitable are driven by perverse incentives to be corrupt, the post office doing more business is hard to see as a bad thing.

Also as many others have mentioned, the Chinese shipping rates were much lower than domestic rates, to the level that cheap products made in the US couldn’t be given away for free cheaper than things could be imported from china. $0 + shipping cost more than similar products sold at a profit and shipped from China.