If you want seller protection (delivery confirmation) to China, USPS Express Mail is the cheapest. But that costs $27-$30.
There is a better way.
USPS Priority Flat Rate Envelope, along with eBay shipping and eBay ShipCover Insurance will get you cheaper protection. The total will be around $13-$15. There is no tracking so if the buyer claims non-receipt you will be giving a refund. The upside is you can then file an insurance claim and get back everything. This has worked for me.
I ship anything ~$75-$249 with USPS Priority Mail, and use ShipCover. $250 and up I use USPS Express. Always remember though, it MUST be Flat Rate in both cases, ShipCover does not work with anything except FLAT RATE.