There is an international agreement to deliver postage paid parcels from another country if the postage was paid in the starting nation. Some countries benefit

There is an international agreement to deliver postage paid parcels from another country if the postage was paid in the starting nation. Some countries benefit more if they send more packages than if they receive. China govt subsidies the post in China and allows cheap or free shipping to manufacturers as it encourages manufacturing. So in part your domestic mail carrier absorbs some of the cost and the Chinese government gets the rest.