When your plan covers 100% of drug claims, and you pharmacist advises you that you still owe money, you need to ask the pharmacist for more information.

1 - Check your plan coverage. You may have some coverage details which may limit the maximum dispensing fee allowed, or add a deductible to your annual costs.

2 - If your plan still says 100%, then there may be more to the story.

When you submit your prescription to the pharmacy and they key in the coverage, pharmacists receive immediate notification from the insurance carrier of the charges.

In some cases, this is due to the fact that insurers will only pay what is "reasonable and customary" charge for that drug -- another way of saying that they will pay the fair market price of the drug. It's a little known fact that pharmacists are allowed to charge any mark-up on drugs they choose, just like any other business - this is not necessarily government regulated for your plan.  

If the pharmacist charges over and above the "reasonable and customary" charge, then this cost is passed on to you.  Speak to your pharmacist about ensuring that they do not charge over and above your carriers coverage. If they won't reduce the charge, you may want to research and price-compare with other pharmacies near you.