I don't see what's so offensive about "That's it?" It's just another way of saying "Do you need anything else today?" If I were shane I would've acted the same way to some pric making a big deal out of nothing.
I've been thinking the same thing. Shane's "That it" is about ten times less annoying than Yankee's rant on here. He's actually managed to make the useless store clerk into a sympathetic figure.