That's not just subsidies, but also how trade routes have been optimized. China, the US and every other country, have non-symmetric mail dispatch systems, optimized for the China->wherever route, with mass dispatch from China branching out to everywhere else. The reverse wherever->China is how much shipments cost over non-optimized routes around the world.
Stopping the subsidies won't stop the differences in volume and optimization (but it can be argued that subsidies no longer make any sense).
I'm sure I am missing something, but surely every container ship that leaves china with a huge pile of mail has to return again to collect more - can't they just take stuff with them?
I don't know anything specific, but a few things come to mind.
First, it's not necessarily the case that these ships are traveling in direct A → B → A... routes, they could be going from China to the US to South America then back to China, carrying some proportion of finished goods and raw materials along each leg.
Second, having to fill up the containers with something at the origin, and unload all that something at the destination, are both huge logistics efforts to manage. Not at all 'free'.
Is a good question, and I don't have an informed answer, but I wonder the same. Maybe the ships leave the US full of exports bound for other destinations in a global "round robin" style route or something along those lines? Or perhaps it's just much faster without the added mass? Would love to hear more about this from someone informed on the topic.
Stopping the subsidies won't stop the differences in volume and optimization (but it can be argued that subsidies no longer make any sense).