Wal-Mart Stores (NYSE: WMT) just teamed up with Alphabet (NASDAQ: GOOG) (NASDAQ: GOOGL). The Google Express service already featured 46 retail partners, but none with the market heft of Sam Walton's shopping empire.Now that Wal-Mart is aboard, does that make Google Express a serious challenger to Amazon.com (NASDAQ: AMZN) and its voice-shopping tools powered by Alexa and Amazon Echo?Let's find out.Continue reading