[diogenescynic]

Most Chinese sellers use a service called Epacket to send small items to the US. Epacket is a trilateral agreement between USPS, eBay and China Post that allows Chinese sellers to send items to the US in bulk at a low rate. Even without Epacket, Chinese postage is a lot lower than in the US.

[Scoundreller]

I think there's more to it than that since the shipping is usually free worldwide, not just the US. I think most countries in the world have an agreement through the International Postal Union to pay by total weight for international packages, and the rate is low. In China/HK and some other places, they pass that low cost to the consumer, unlike the US.