Lately the hottest thing in e-commerce is “ClassPass for X,” a trend that combines monthly subscription fees with experiences. (Millennials love experiences, I’m told.) It follows the success of ClassPass, a well-funded startup that sells unlimited fitness classes at participating studios for about $100 a month. There is already a ClassPass for hair blowouts (Vive; $65 per month), massages (Zeel’s Zeelot program; cost varies), and live music (Jukely; $25 per month). It’s too soon to call this new model a fad, but if past e-commerce innovations are any indication, it may not be long for this world.