Unfair Practice

I registered on the Builders Warehouse website to make a purchase, I completed my purchase and used my card to pay. After the purchase I got an email to say I will receive a 15% discount when entering the code. I called your call center to apply the discount but they say they would have to cancel my order and then refund my money after 5 days then I would have to make the purchase again. This would not work out for me as you guys only have 4 items left in stock of this item and that would mean I would miss out on the deal. Please apply my discount to my existing order as you guys promised a 15% discount. Order number ***