It's not simply about not affording something, it's about keeping track of spending, so you are aware of the moment you go from 'I can afford this' to 'I can't afford this'. If you've got a tenner in your pocket at the start of the day, you know when you've spent it. If you've got a card, you spend that ten pound, you still have the card. Did you get 6 of those £1 things, or 7? Do you still have enough left over for a pint?
And if you pay by phone, you should have an app that allows you to see how much you've spent, just like looking in your wallet!