Management fails staff

Going to Thailand next week so I picked up a sunscreen of a french brand BioNike as it says it's for sensitive skin. Then I notice a sign stating that you get a free beach bag if you buy two products. Awesome! I read carefully as you can always get caught out by fine print. Get to the till ask for my bag. Cashier calls the supervisor, supervisor tells me you get a free cleanser, not a bag. I go all the way to where the promotion is and bring back the sign where it states clearly you get a bag. She mumbles what I think was an apology and leaves. Another supervisor comes along and checks to see if I have the correct products, which I did. She goes away leaving me AND the cashier rolling our eyes. I finally get the bag along with a soft 'apologies for the inconvenience'. Unfortunately for them I am one of those customers who insist on what is rightly mine, even though everyone is staring. Luckily for them I am fair and am not too loud and unreasonable. But for that time where someone is, please can I ask management to have a quick status meeting with supervisors when there is promotions to let cashiers know so that customers aren't standing around wasting everyones time.