Overcharged for medical

My wife needed to get her eyes checked, as most people do.  Our insurance provider requires us to pay up front, then submit receipts for reimbursement, which we did.  Yesterday, my claim statement came back.

We are not able to pay the full amount of this expense because your plan covers it only up to the amount typically charged by health care practitioners for this service. Your practitioner has charged more than this amount.

WTF?  Am I now required to shop around for medical care before I get it?  Perhaps I should ask my doctors “how much are you charging?  Good.  How much are you supposed to be charging me?

I don’t know how I was supposed to know the “going rate” for an eye exam, but I definitely know now.