The neighbor had his kids return about 7 cartons, had to see the kids trying to hold them. I stopped the kids and said "hey, I'll show you a trick" and showed them how to stack them. They said "that would have been easier"
I find they don't always stack nicely. Every brand seems to have a slightly different style of carton, so while the egg part always stacks, sometimes the tops stack inside each other neatly and sometimes they don't.
Heck, I'd just like for more of mine to bring the darn things back! Maybe if you set a few out where they can see them when they pick up the eggs they'll figure it out?
This might sound weird but if I were a customer who planned to return used cartons knowing they would be re-used again I might hesitate to stack them that way. I'd be afraid it might be unsanitary, since the bottom of the cartons lie on the refrigerator shelves, the counters, etc. And then those dirty bottoms would get pushed into the interiors of other cartons, etc. They may be trying to be courteous.