In a world where nearly every transaction flows through digital channels, integrated payment systems like those of American Express Company (AXP) are the backbone of financial operations. The legacy financial giant known as AmEx has successfully adapted to the age of digital payments, with its reputation further bolstered by Warren Buffett’s long-standing investment. Regulatory filings show Berkshire Hathaway (BRK.B), run by the legendary value investor, holds a staggering 151.61 million shares of AmEx worth about $35.1 billion, which accounts for 12.5% of Berkshire’s stock portfolio. Buffett’…