Venting helium allows the lift to be controlled and the balloon to be overinflated for a quick ascent and then to vent helium to “hover” at altitude
Polypipe fittings are much lighter than copper ones, also the valves have all contact surfaces made of carefully machined plastic, so should be more reliable at low temperatures than rubber and copper based valves. In the UK you can pick up polypipe at Wickes. This design using a polypipe fitting can be built quickly and easily for under £20, but the small diameter makes it impractical at very high alt > 24km. It can be built into the payload and doubles as a foolproof inflator. weight approx 300 grams, but using different plastic pipe or reducing the lenght of the 15mm inflation/vent pipe could get this down.