American Express Co. won’t charge small businesses to use a service for creating discount offers that are delivered to Amex cardholders via a new smartphone feature and other platforms as it looks to expand merchant acceptance of its high-end cards.Small merchants represent the last mile for credit-card companies, which want as many retailers as possible to accept their plastic to increase transaction volume. American Express enjoys mass acceptance among major merchants such as Best Buy Co., Wal-Mart Stores Inc. and Target Corp., but has been less present among small businesses, which often scoff at the fees they must pay each time a customer swipes a card.