Learning the ways of shopping

Zac has been enjoying paying for things for a month or so. He understands the concept of handing money over and accepting change, or handing a card over, punching buttons on a machine and taking the card out of the machine (he is not yet punching the buttons – there’s only so much goodwill on sales assistants’ part that I bank on).
Today, he bought his first book. Chose it, paid for it, and wouldn’t give it to the sales person to scan – he understands that he keeps the item chosen!

Here’s the money

What do you mean you want the book as well?

No, I know how this works. You keep the money, I keep the book!