In September, AARP, the giant organization for older Americans, agreed to promote a burgeoning chain of medical clinics called Oak Street Health, which has opened more than 100 primary care outlets in nearly two dozen states. The deal gave Oak Street exclusive rights to use the trusted AARP brand in its marketing — for which the company pays AARP an undisclosed fee. AARP doesn’t detail how this business relationship works or how companies are vetted to determine they are worthy of the group’s coveted seal of approval. But its financial reports to the IRS show that AARP collects a total of abou…