Shopping does not always involve common sense

Jman and I went to the grocery store today, to do our bi-weekly shopping. Jman wanted to buy a pack of gum and had brought his own money, so I told him to go ahead and pick some out. My shopping cart was rather full, my cashier was a wee bit slow, and I did not want to have to wait for him to be rung up after me, so I told him to go over to the self-check out lane to pay for his gum. I noticed that he had a pack that was on sale, but he needed my shopper card to get the sale price, so I gave it to him (it was on my key chain).
I finish getting rung out and notice that Jman still has not returned. I can see him at the self check-out, which now has an incredibly long line all standing behind him. I wheeled my shopping cart over as I see the attendant walk to speak to him. As I get into hearing range I hear them ask "where's your mom". I speak up, and they explain to me that he is trying to pay for his gum with the shopper card. I laughed, since it did not even occur to me that I would have to tell him what it was used for, since he has been shopping with me since he was a baby and I always use a shopper card. Of course, how would he know, as he explained it, he doesn't pay attention when I ring out my groceries. Yes, he has seen me use the card, but he did not realize that it was not like a credit card. "You should have told me Mom, I didn't know!". Of course, I bring up the fact that when he walked over the lane he had his money in hand and why would he think that I was going to "pay" for his gum with my "card", and why would I send him to another shopping lane if I am going to pay for it anyway, wouldn't I just add to my cart of groceries?
I did feel bad for him, since he was embarrassed that he did not know better, but honestly, would you have thought that you needed to tell your child how to use the shopper card? What other "common sense" things do I need to tell him about?