It depends on their contract with your insurer and your policy. In network benefits may require that they cannot charge you more, which would be breach of contract w/the insurer or fraud possibly, but some out of network benefit stipulations explictly state you are required to make up the difference between what they charge and what insurance pays.

Best way is to look up your plan by googling or thru your account. Calling them is a good idea too, but they will often give you different answers to the same question. In my policy, it's clearly stated in the benefits book that I get online.