Clicks 3 for 2 special did not register and gaslighted by manager

This is not the first time it’s happening and many other people have told a similar tale, where Clicks has a 3 for 2 special but it does not register at the till. My mistake for not checking my receipt at the store and apparently for taking a pic of the very visibly displayed “3 for 2” sign. So I contacted clicks customer care and got a call from the manager only to be gaslighted! Yes, I was told the special was not for the items I picked up, yet the special was there online, it was there in store and the sign even had pictures! So how did I see and read wrong and I even asked my niece to read the sign carefully to make sure we’re taking the right products. Only to be told the phone to go back to the store so they can see what they can do, all whilst telling me it was the wrong product and the name given was a completely different one, yet the one I tool not only had the name, but pics too. I refuse to waste petrol only to “see what we can do”. At this point the only rectification that makes sense is to return the price of the item that should have been discounted back into my clubcard, which is actually their favour since it ensures I spend my money there which I could have spent elsewhere.