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).