Five years ago, the world's largest retail chain didn't have to worry much about the world's largest online mall. Only about a quarter of Wal-Mart Stores customers shopped at Amazon.com, according to data from researcher Kantar Retail. Today, however, half of Wal-Mart customers say they've shopped at both merchants. That's leaving the mega-retailer with a massive online competitor that is too tough to ignore. In the last year Wal-Mart has increased its investment in its online business. The company has spent more than $300 million acquiring five tech firms since May and hired more than 300 engineers and code writers in the US and India. Wal-Mart is trying Web-based shopping tactics, like its Pay With Cash program for Wal-Mart customers who don't have credit cards. The new program allows them to reserve products online and pay cash at their nearest store.