Here is how much a regular package goes from China post:
If I sent my phi-2 20X4 kit (5 oz) from China to USA/rest of world, I spend around $6, half of what I pay if I send it from USA to China/rest of world. So before USPS price hike they were in par. This is what a typical person pays. As far as I know, China post is also doing very poorly, due to internet (no one is writing letters anymore), and lots of private local and regional deliver services, even private international ones, through which most these free shipping packages go. You will find an NY or CA address on the package cause the package was shipped to NY or CA distribution centers and then cheaply shipped to you via USPS (commercial rate, not our rate).
You also can't exclude some sellers trying to make themselves good names by charging too little.