Walmart Inc. has begun telling online shoppers that some products in its warehouses are “out of stock” after the retailer changed its e-commerce systems to avoid orders deemed too expensive to ship.

Amazon.com and Sen. Bernie Sanders (I-Vt.) are sparring over working conditions at Amazon warehouses, after the senator called for legislation that would require large employers to reimburse the government for food stamps, Medicaid and other federal assistance received by their employees.