How is it possible that Chinese vendors are able to ship a small / light $3 part to a doorstep in the US for free? When shipping items the opposite way via USPS it is often greater than the cost of the part making it cost infeasible to return. Is shipping somehow subsidized?
The major reasons that I understand are that:
the Chinese government subsidizes shipping.
the US Postal Service, eBay China, and Hong Kong Post office have a trilateral agreement called the ePacket service which roughly speaking, allows shipping of small packages to the US at bulk pricing (instead of at the price for individual sales).
I don't think it's common for there to be "free" shipping though.