Amazon this week expanded its discounted Prime membership program, already available to EBT card holders, to include Medicaid recipients. Eligible customers can become Prime members for $5.99 a month, less than half the regular $12.99 monthly fee. The move — another step in Amazon’s efforts to reach underserved economic communities — ratchets up its competition with Walmart. Amazon provides customers with a link to authenticate their membership in eligible government assistance programs. It is not clear how the company has been working with the Department of Health and Human Services to verify the records.