Retail Shopping...

Now, these are not immutable, there are exceptions, etc. Take from it what you will, which is hopefully a not of respect for employees and also for fellow shoppers. And before anyone complains that this goes against "customer service": I have worked various aspects of customer service for about 15 years now, everything from lowly bagger to manager. A lot of these benefit not only the cashiers and employees, but you as a shopper. They can help keep costs and prices down, and it keeps you from being a douchenozzle to your fellow shoppers. I will admit, though, that yes, it will also keep a cashier from wanting to "accidentally" bag your eggs under your 10 lb bag of sugar...

Retail Shopping Etiquette

1. If you get an item from a freezer or cooler, it likely requires cold. If you change your mind on that item, put it back in the cooler or freezer. Likewise, if you decide you don't want some item that you picked up from a shelf, do not put it in the freezer. There is no reason to toss produce or cans of soup into the store's freezer, nor should bacon be found sitting in a box of candy. This is as much for your benefit as the store. It reduces the amount of food that the store throws away as waste, which reduces the chance that they have to raise costs. And its safer, because no one wants raw meats and such just randomly hidden behind other groceries, off refrigeration.
2. Two notes, if you go to a store intending to write a check for your purchases. First, go ahead and fill in the things that won't change. Filling out the store name, date, even signature, while the cashier scans your items speeds things up for everyone. Secondly, if you intend to write a check, bring a pen. Bring a damned pen. A cashier may not have one, because they don't often have need for one. A cashier might have a nice pen, that they don't want to risk someone forgetting to hand back. You walked in wanting to write a check, knowing you'd be paying in a way that requires a pen. Bring one.
3. Not every purchase needs a bag. Skipping one saves the store's costs, which keeps their prices low. Milk, for example, does not need a bag. It already has a handle. And saying that a bag makes it easier to carry does not actually fit logic. The milk handle is wider, distributing the weight wider, and the handle is far less likely to break. If the item is so big that nothing could fit in the bag with it, using a bag is kind of wasteful.
4. If you use a basket, drop it back off on your way out. Chances are, the way most stores are set up, you will be walking right back past the racks where you got them from. While the cashiers could put them back if you leave them somewhere random, they may not be able to do so immediately. Until they can, these baskets get in the way. It becomes a safety issue, as I have seen people trip over them. And when the cashier finally can take them back, they have to leave their register to do so, which means that they are not there to serve others due to your laziness. Now, granted, if you have 2 or three bags worth of purchases, plus a bag of dog food in your other arm, it's understandable if you can't carry it. For the most part, though...
5. As in number 4, if you use a cart...put it back. You will be exiting right past where you got it from, and these take up even more space when you just randomly leave it at the end of the checkout lane. It just makes you look like a lazy, entitled prick as every else has to go around your detritus.
6. At least in America, almost every store signals whether a register is open by using the same signal. Wal-Mart, Kroger, Home Depot, Ingles, Publix, Target, Dollar General, all of them. If the light is on, the register is on. If the light is off, register is closed. This is a simple thing, and while occasionally a cashier might forget to flip the switch, this is a general rule of thumb. If you see the cashier ringing out someone, and the light is off, don't blithely start tossing your cart's contents onto the conveyor. Ask if they're open, or just go to another register. By the same token, if the light is on but no one is there, chances are that the cashier simply had to walk away for a moment, to reshelf an item that some other dufus left behind. Give 'em a moment.
7. There should be little or no surprise when you see our total, no reason to tell the cashier “stop when I get to $X”, etc. Prices are generally right there on the shelves with the products. Phones come with calculators built in. Scrap paper is easy to get. If you have a third grade child, chances are they can track your total as you go. Use these resources. When you have to drop off a dozen items because you grabbed more items than you have money, it shows you as a lazy schmuck, these items get in the way of others, and the cashier will likely have to walk away from their register to put these items away.
   
   
   Lastly, remember to please keep calm. If you start yelling because of some small error, chances are that you are yelling at a person least able to help. And we talk. We pass stories. If you're a dick at one store, other stores, even competitors in the area, will know who you are.