In general, I believe that the cost of this the "membership" is not a qualified medical expense. The 15% off additional services, no-cost for x-rays needed for reasons other than normal course of preventive care and the reference to this being a"membership" means that you are paying to receive discounted prices, not to receive a specific treatment.
If your dentist provides a breakdown of the portion that is payment for the cost of the regular preventive care and the portion that is not, you might be able to treat the portion for regular preventive care as a qualified medical expense.
