Right, but helium doesn't just magically "go up". It provides lift in proportion to the difference in density between the gas in the balloon and the surrounding

Right, but helium doesn't just magically "go up". It provides lift in proportion to the difference in density between the gas in the balloon and the surrounding atmosphere. As the atmosphere thins, the amount of lift capable of generated by a helium balloon decreases to the point where, at the altitude you've mentioned, it could lift a total of about 0.8kg (according to GP's calculations). The point being that you wouldn't be able to have much of a payload lifted to those heights.