Some ATM machines will sell you stamps. Others will not. 

When paying for an item, the cashier may hand you a hand-held device with colorful buttons on it, connected to a piece of telephone coil-cord and you are expected to enter an electronic number that will pay for something and you will know what to do because you've seen it done before. 

I am special kind of machine that drinks a lot of coffee. And I do this because I've always done this, or because my father does this and I am well-trained in it.