Nice game, the design is well done! I think you could maybe add shortcuts for the stamps to make the game a little faster, its too difficult to stamp 4 or 5 parcels before the next badge gets in. (then again, maybe i am just too slow :D)
Also, even when stamped green correctly (= correct weight, not damaged) it plays the error sound, maybe you could display the score for each parcel so players understand better if they understood the rules correctly or not.