I sort of see that BUT....the same packet is travelling . It matters not which way it travels at some stage it hits the Royal Mail.It is (or at least was) subsidised by the Chinese government - but not to the levels I've seen postulated...
From what I've seen, their postage is cheap to us, but not necessarily to them.
As a percentage of income, it would appear that to send a letter it costs your average Chinese person roughly about the same as 'us'. It's our income being so much higher that makes their postage look so cheap.
It has to be a case of who is responsible for mail in the other country.
Its all a bit weird as its privatised.
For example a Parcel sent via DPD gets moved by DPD or a subcontractor where ever it is.....china post reaches here and who moves it....Royal Mail......but why...who pays them?