Shoppers Drug Mart complaint: Didn’t receive the discount for seniors on seniors day mar 4 2021

I purchased items some on sale some not on sale totalling 75.45 .when I paid the clerk gave me the 10$ coupon so I assmumed I received the 20% discount on non sale items
When I looked at my bill when I got home I realized I didn’t receive the discount , phone customer service at the store ,talked to a pleasant person who got the info off my bill and agreed I didn’t receive it
She told me to bring my bill into store next time I came to have it corrected
I went in the next day and was informed not by the pleasant cashier but her supervisor she called , that I would need to bring the items into the store to get the discount , also that since I got the 10$ coupon, the bill must have been correct . I said why wasn’t I told that in the first place , I had put all my items away , so I said I couldn’t be bothered
I wasn’t too happy and decided to complain as I have dealt with Shoppers for years , get all my prescriptions from them and felt I should have been better dealt with