BENTONVILLE, Ark. — Walmart and Humana have teamed up again to help Medicare beneficiaries afford their prescriptions. With annual enrollment open until Dec. 7, 2012, for 2013 Medicare plans, the Humana-Walmart Preferred Rx Plan announced that it offers one monthly plan premium of $18.50 a month for beneficiaries in all 50 states and Washington, D.C., regardless of where they live.