Amazon.com Inc has made a formal offer to buy a 60 percent stake in Indian online retailer Flipkart CNBC reported on Wednesday, citing sources.Amazon also offered Flipkart a breakup fee of $2 billion and is likely to be on par with Walmart Inc’s bid for the e-commerce company, CNBC TV-18 reported.Reuters in April reported that Walmart was likely to reach a deal to buy a majority stake in Flipkart by the end of June.Representatives at Amazon, Flipkart and Walmart were not immediately available for comment.