Um if I may suggest something, I had the same problem, what I normally do is (do not merge the png with the backround until AFTER this is finished)... I use the wand tool and click on the shadow (make sure it's in the shadow and not the backround), then I just use Brightness/Contrast to make it darker. I guess if the shadow were more complicated it may not work, but it always works for me, just DO NOT use the "color picker" tool, it will select EVERY part of the image that has the shade of grey that the shadow is, so besure to use the WAND TOOL.

Hope that helps.