BN have even got a shaker where you can have the powder in the base and liquid in the main chamber. Screw off the base, toss in the powder when you are ready. Or just use a normal shaker and add water at the gym.
WPI is a highly processed form of powder without all the lactose and stuff you get with WPC. On a hot day you are better off mixing it right before you drink as it will break down fairly quickly on a warm day. Take a sniff of your empty unwashed container after a few hours in your gym bag.