Buyers are snapping up tens of millions of stamps commemorating the late Princess Diana, and British postal officials predicted the set would be the biggest-selling ever. Among the customers for the opening of sales last week were an American who bought 900 sets and a Japanese tourist who spent two hours addressing copies of the special stamp presentation packet to send home. Some bought sheets of the stamps to frame. The set of five first-class stamps will be the only one issued in Britain bearing Diana’s image. Each stamp costs about 40 cents.