If your insurance plan requires that you pay a copay for each dr visit then it is actually illegal, or at the very least against the contract, for the provider to NOT charge you the copay. However, copays are supposed to be collected at every visit, at the time of the visit. Not billed 18 months later. I would call the provider and ask why you are now being billed for the first time for copays that you should have been made aware of at your visits. If they don't have a very good answer you may want to tell them that you plan on contacting your insurance company to see if their practices are legal. Your insurance company would be interested to know that they are not charging copays until you leave the practice.
Also, there may be an explanation (although I'm finding it hard to come up with one) for why you weren't billed or advised of your balance sooner.
Good luck
Michele