Cashier being rude at Uni-mart?

What they did WAS rude, but your response was just as childish. You shouldn't have gone to the other store and then taunted them with the food you bought. If you had asked to speak to a manager and stressed that you were upset with the workers, you probably would've gotten a discount or something, especially if you said you were probably never going to shop there again. In addition, the workers would've gotten in trouble for being rude to you, and they might have even been fired. So, you weren't really wrong in what you did, but you could've made a better, more mature choice, and possibly even ended up with free food AND revenge.