And how often do you stop your checkout clerk and tell them to stop scanning because you've exceeded N number of dollars? Once you've pulled your cart up to the checkout line, I'm betting you've most likely settled on what you want to buy.
I've seen people do just that often enough. They'll sometimes sort their items in order of descending importance, and ask to stop when the total exceeds some amount.
I don't want to say the "p" word and spark a huge social justice flame war, but a lot of comments in this thread are so obviously speaking from a place of financial advantage.
In response to your comment in particular, yes, I have done that. Not often, but it has happened often enough, and only a few dollars each time. Do you think people whip out Excel and keep a running tally of every item in their carts?