It's not magic, it's just that the USPS doesn't have to do the work of sorting them. Labor costs in China are much cheaper than in the US, it stands to reason

It's not magic, it's just that the USPS doesn't have to do the work of sorting them. Labor costs in China are much cheaper than in the US, it stands to reason their cost of sorting there will be less than what the USPS implicitly charges for sorting here.