If it isnt metal the NFC should work. I bought a couple covers from China and one was metal and i didnt expect the NFC to work and it didnt. The other has leather on the back and NFC worked fine. Not sure what your talking about but you have to know what your getting and what works with NFC And they fit just fine. Its $6, what do you have to lose?
There are bigger batteries you can buy too but you have to make sure they are NFC enabled.
I understand the skepticism but ive bought a decent handful of things from China & Hong Kong and they were fine. Hey im just trying to save you some money. Im one that doesnt think the covers we get from here that are $19-$29 are any better. Still made in China.
Here is a site that is legit ive bought from and they have all kinds of stuff.
http://www.eforchina.com/list/2442/Former-Shell---Back-Cover.htm