It happens all the time. One minute you've never heard of an up-and-coming startup, and then suddenly it's everywhere. This appears to be the case for the millions of users actively flocking to the viral shopping app Temu. Temu isn't the first app of its kind, but it's quickly becoming the most popular. Previously, Alibaba Group Holding Ltd. grew in popularity by allowing U.S. consumers to buy cheap goods directly from Chinese manufacturers. But issues with customs, hefty shipping costs and unre