Amazon has taken another step to woo low-income shoppers to its site and away from rival Walmart. The online retailer opened its discounted $5.99-a-month Prime membership on Wednesday to people on Medicaid, giving it an even bigger pool of potential shoppers who may otherwise have been unable to pay the standard fee. Since June, Amazon has offered the same Prime discount to people using food stamps or other government assistance through an Electronic Benefit Transfer card. Adding Medicaid recipients makes Prime, and its expedited shipping for no extra cost, accessible to more people, Amazon said. Industry analysts believe Amazon is...