American Express Co. Monday reported a net charge-off rate on its U.S. credit cards of 10% of managed receivables for May, down from 10.1% reported for April. The rate in May, however, remains more than a full percentage point higher than the 8.8% reported in March, according to a company filing with the U.S. Securities and Exchange Commission. AmEx's 30-day delinquency rate, which is trending downward, fell to 4.7% in May from 4.9% a month earlier and from 5.1% in March. In March and May, AmEx said it sold some charged-off debt to third parties, and it treated the proceeds as partial recovery. The company said it intends to continue this practice as part of its strategy to recover value from written-off loans.