A Squared Away reader wrote recently that he and his wife saved $2,400 a year by paying cash for their medications. When a pharmacy sells a prescription drug to a customer, the health insurer reimburses the pharmacy at a negotiated rate that covers its cost for the drug, its dispensing fees, and any additional markup. It’s often the case that a patient’s copayment exceeds the pharmacy’s reimbursement, resulting in an overcharge in the copayment. More than one in four copayments were overcharges in a March analysis in the Journal of the American Medical Association of some 4,000 outpatient drugs and 9 million insurance claims by people of all ages. We asked Mohamed A. Jalloh in Napa, California, to guid…