OMAHA, Neb. — Billionaire Warren Buffett’s company has been adding more Amazon shares to its investment cart. Berkshire Hathaway said in filings with the Securities and Exchange Commission on Wednesday that it owned 537,300 Amazon shares at the end of June, up from 483,300 shares. Buffett first revealed Berkshire Hathaway’s stake in Amazon in May, although he said one of the company’s two other investment managers made the pick. Investors follow what Berkshire buys and sells closely because of Buffett’s successful track record. Berkshire officials don’t generally comment on these quarterly filings. Besides the Amazon move, Berkshire also increased its Bank of America stake slightly in the last quarter, and it added to its US Bancorp stock holdings. In addition to investments, Berkshire owns more than 90 companies in a variety of industries.