This is entirely correct actually. Number of items on day n = (13 - n)*n

It’s not just a sale. Gift card money is invested and the company makes returns off of it, and all they have to do is provide you the base value of the gift card in coffee or whatever at some later date. Plus, if your purchases don’t add to a whole number, millions of gift cards with like 30 cents left over in each of them is a ton of free money for the company. Gift cards are a huge scam