I remember that some BK mama can buy it with 30% off war. But they also mentioned that some branches said that it's 公價貨品, so no discount law. Which branch are you going?
Am going to Manxxxgs at Kwai Fong / Metropolis Plaza today and found there are few selective Mustela items have special discount.
Baby Hair Shampoo : 53.90
Baby Lotion : HKD107.90
Baby Powder : Could not remember the exact price after discount and it should be around HKD50-52.