I went to a large chain drug store this week in Manhattan and tried to buy 4 boxes of their house brand.
I wasn't carded, but the cash register bell went off and it refused to put the transaction thru. The cashier was befuddled, but called her manager over. I probably could have bullied them into splitting it into two transactions, but I didn't have to -- before I could say anything the manager instructed the cashier to ring up 3 boxes and then start another transaction.
The cash register would have been quite annoying if I was trying to buy 40 boxes.