McDonald’s complaint: cheated me out of a promotional product

I went the restaurant to get the buy one double burger get one free from app. I had trouble with app, but managed to navigate what I thought was success at the register with clerk, my order came up without the free promo burger, the clerk ask the lady who filled the order to help me, and explained what we were doing, she was not helpful at all, said nothing she could do, I know its only $2 im upset over, but its more the way it was handled and no one in charged seem to care that i was not getting what I came in for, no one semed to care one way or the other, she said I cannot refund it, I said I dont want a refund, I want the promo I came in for. No help was offered, I left mad, and now after 50 years of success with Mickey D, I feel that they dont care about my business any longer, what should they, huge corporation with plenty of business, who am i , no one to them.. I cannot return to do business with someone who done me wrong.