$50.19 is, according to the chart, the biweekly rate for self-only under the American Postal Workers Union plan.

Yes, legislators can select any plan, under the same procedures as other Federal government employees. That particular one, like others that are sponsored by an employee organization, requires that the individual either be a member of that organization or pay an additional fee to become an "associate member". Those vary from one organization to another, and I don't know how much they are without checking each plan's brochure (I checked APWU - it's $35). Under the law (5 US Code 8901), "employee" for the purpose of health insurance includes both Members of Congress and congressional employees.