Using Insurance at Walmart Vision

I just left Walmart Vision Center. I was trying on eyeglasses frames at Walmart — they have some cute ones. Found a frame for $88. My vision policy covers $150. An employee just told me that even though the policy covers $150– Walmart corporate has directed employees to tell customers that they can only bill the provider for $55 (for any frame). She didn’t even ask which VSP. She just said max billable to VSPs for frames is $55. Please someone explain this!!!