When you get to the cash register, what do you do first?
Do you imagine the balances due on various credit cards, and choose the one with the smallest outstanding debt? Do you mentally compare APRs and make the optimal financial choice? Do you calculate whether you actually need the product in your hands, versus its cost, versus the lost opportunity of using those dollars on something else?
Or do you just grab whichever card is on top of your wallet, and spend?