Can I use iTunes credit to help subsidise buy hardware in Apple online store?
Unfortunately, no. One can only use iTunes credit to buy iTunes Store content and apps, not hardware. I think that it says that on the card itself in fine print, but I could be wrong.

I couldn't find an official statement from Apple, but here's something from the Apple support community: https://discussions.apple.com/thread/2592711?start=0&amp;tstart=0