What could I do when I was billed for the service I already paid for?

When scheduling me for cataract surgery, the surgical coordinator (KT) told me that Medicare doesn't cover my doctor-recommended surgery with insertion of a multifocal lens, and I had to pay out of my pocket for the service before having surgery: $3200 for each eye, and that's all I had to pay. But after surgery, I received a bill for the service. When I complained with KT about being billed when I already paid for the service, she checked my account and told me that I was fine, the balance on my account was zero. But they continued to bill me even after I sent them receipts of my payments, and said that my payments were already applied to the lenses and "non-covered item or service" (I have no idea what this is), none to the surgery itself. I don't knnow what to do now. I'm 81 years old and only want to live in peace. Thanks for your help.