You must not shop at Walmart. Say what you want about Walmart, but they get you out in a hurry. I can't remember ever being asked by them about some extra thing at checkout. If you just want a couple of things you can just use self-checkout and you don't even have to talk to anyone at all. They don't even want a signature unless your total is above a certain amount. They even generally avoid helping you if you're clearly wondering around looking for something -- who can beat that?
...I pretty much just shop at Walmart and HEB but when I go into the local ACE hardware they try and guilt me into rounding up to the nearest dollar for whatever their cause is and I usually submit. And when I'm looking for something ACE employees sometimes even bother me, asking if they could help and all. NO THANKS, I want to find it myself and your extra bit of customer service doesn't make up for your checkout guilt!