Perhaps you’ve had a mammogram recently, or taken a child for an immunization or consulted with a specialist about a weight problem. Since late 2010, those visits to health-care providers have carried an additional benefit: They’re free. Under the Patient Protection and Affordable Care Act, signed into law 28 months ago and largely upheld in June by the Supreme Court, it’s illegal for insurers to charge consumers a co-payment for a long list of health care services designed to prevent disease.