Domestic mail is like sending separate items, which get bundled together, then un-bundled again to deliver them to their final destination. All these steps happen at domestic wages, and at best with medium volumes. If there are not enough items to fill a container, it's still getting sent some other (more expensive) way.

Items from China get bundled by lower wage workers, the bundles are sent by lowest priority freight (usually a 7-45 day delay), bundles are only sent when fully filled in order to minimize costs per item, and sorted to minimize un-bundling costs.

This won't change no matter what, it's just economy of scale.

There are other factors like the reduced shipping costs for China set by the UN, or the fact that some will ship items at a loss just to convert Yuan to USD through Hong Kong, but ultimately the production vs. consumption imbalance is what decides most of this.