Hey you did it, congrats !!
Definitely a shield bonus is a good idea and I'll add it in next version !
I also like your suggestion of adding some rewards on levels 4 and 7 ! (and maybe some "gravity blasts" and shields here and there)
But I think the rewards must be gold ore rather than diamonds otherwise I fear to unballance the shop.
Talking about ballance, I have the feeling that after some levels the shop tends to loose it's interest because you don't have anything else to buy did you have that feeling too ? I was considering making the golden skin very expensive (like being an hard achievement) and activating a little percentage of the "bonuses attractor magnet" permanently (except in the cave level) when you buy it...