Dick's Sporting Goods review: Worse experience with the store cashier

Purchase multiple items at the store today, everything was going great till at cashier. I understand store closing in 10 min. but it's not an excuse for not helping customer correctly. After Casher ring my purchases, I informed the cashier I still have a gift card with some credit first, after the cashier inputted that, I mentioned to cashier that I also have $10 Store Award along with 20% coupon, the cashier just simply told me that I cannot do that since he has already inputted the gift card and clicked done. So, I asked the cashier if he can simply cancel the order and redo it again with Award and coupon code. He just simply tells me that he cannot cancel my order and I will need to come back next time to ask manger to credit me back? So I told the cashier that then I dont want to buy it then, he's like okay then I can cancel my order and walked away?