Child doing instacart shopping

I had an odd interaction that I just wanted to put somewhere and I assumed this would be the place.

TLDR: A kid came into dollar tree to shop an instacart order by himself.

I work at a Dollar tree, just an associate cashier. One evening recently a couple hours before close I had a kid come into the store alone. Maybe like 12/13? And that's being generous. He grabs a shopping cart and starts shopping, looking between his phone and the shelves. It was weird to watch causes all his mannerism were very 'adult on business' rather than how kids usually behave in the store. I don't remember anything specifically other than it just felt weird.

I'm keeping tabs on him as he goes about his business just out of concern cause even though there isnt really a rule against kids being in the store by themselves it was after dark and we have a lot of weirdos in our area that hang around the store late at night.

Eventually he comes up to me, holds his phone up and asks "Do you happen to have any of these and if so where can I find them?" Looking at his phone it's unmistakably instacart that he has open. I just tell him where it is and point him in the right direction. "Great, thanks." He goes back to shopping.

I go about my business doing the usual end of night tasks between ringing up the few other customers. Then he comes up to the register, double checking everything with his phone before he puts it up on the belt. And it was quite a few items. I just do as usual. scan, bag. This whole time I'm just mildly weirded out cause this very obvious child is talking, gesturing and just going about this in a very adult way. I felt like a background character in like a reverse BIG situation.

When I was finished scanning his items he loaded the bags into his shopping cart. Tied all of the bags closed and then paid with a card. After I give him the receipt he pulls the cart off to the side and takes a picture of all the bags in the cart. Taps on his phone for a bit then looks up at me and goes "Awesome. Thanks again, have a good night." Then pushed it out the door.

Right after that I went to the manager on duty that night and I told her about what happened. She just said "Oh sick, I would've done that if I could when I was a kid."

I can only assume that the kid was shopping in place of their parent/guardian? I don't know how else someone that young would be a shopper. I didn't do anything about it cause firstly it's not like I have the kids name or the info attached to the account. Second, it's not really any of my business.