Missed the deal

we went magic sauce yesterday .. we ordered 5 burgers 2 chicken and 3 beef one loaded fries and drinks .. i want to tell u guys the taste is all good but when i came back home and check the page i got to know the ramzan deals as its buy 2 burger and get 1 free and 20% off..
all i can say is i dont the exact time when we orderd may b its 8:30 and 9.. they guide us each and everything but didnot tell us about the deals and charged us the regular amount..
i just want to ask that if any food chain introduce deals they always mention it to the walk in customers and charge thdm accordingly na k jis ko ni pata us se pora charge krlia.. seriously this thing dissapoint me so much
there is a suggestion for you guys as your taste is no doubt good but you have to work the above mention thing