Wal-Mart (NYSE:WMT) and American Express (NYSE:AXP) plan an 8:30 a.m. call for a financial services announcement that could point a way for the cash-transaction-heavy retailer to reach out to low-income shoppers with new credit offerings. The two previously partnered on providing Amex's low-cost prepaid Bluebird card, participation that Wal-Mart ended this summer based on poor sales.