Just a quick heads up coming into the holiday season: It costs you nothing to be nice to those in service industry. Remember that the person serving you isn't at fault for there not being anymore advent calendars you forgot to buy before December began. And staring at them is not going to change that. Nor is yelling, or sighing, and huffing really loudly going to make advent calendars who up.
Also, where in the world has being a huge asshole to them make them help you even more? I can tell you right now that if you're a jerk to a retail worker and ask them to look in the back for something, they're going to go back there and talk to a coworker, or stare at the wall before coming out and telling you "no, sorry" and being insincere.
My favourite is when you tell someone they can't return clearance items and they just turn to the person standing next to you (lower in the totem pole of authority) and hope they're going to give a different answer. Guess what? They'll just look to me because I'm in a position of authority and I won't budge. I especially wont budge if you're rude to me or my cashier. So help me god, if you're polite I will do my best to make the situation work for you. If you're not polite, I won't try my hardest to make it work for you. And you'll get to enjoy my RBF in a sincere fashion.