Why did I receive a bill? I already paid my co-insurance/co-pay at the doctor's office.

You may have paid your co-pay at the doctor’s office for the doctor’s services.  However, this statement is referring to a separate benefit under your insurance for the product you received at your doctor’s office.  Depending on the product, your insurance is telling us that you owe a percentage for co-insurance.  The co-pay is the flat rate you pay for the doctor’s office visit.  For more information on co-insurances, see the FAQ, "What is co-insurance?"