This is exactly what we're thinking! A little thingy that floats up and fades away. Almost like... points in a game. The only thing is we're thinking of doing it over the item, so you know that item has been added. Upper right corner, people's eyes may not see (though the movement will probably catch). Something to test!
I think peripheral vision responds better to movement. (Evolution of our ancestors making the peripheral vision less detailed but quicker acting to be quick acting for possible attacks. I could be mistaken though)