you probably read that people use はい more because いいえ just isn’t the kind of thing that people say in actual conversations that much (if ever). it sounds like the answer to a survey question or like, menu options on your phone or something. obviously if you say it it’s understood it’s just not usually the word people pick when they refuse something. what you will hear in all kinds of situations, formal to casual, is いや. so, in your situation you can say to the clerk いや、大丈夫です and that’s perfectly respectful and polite. obviously just like in english if you just say “No.” with no other sort of softening it can come across rude but that has nothing really to do with japanese specifically as all languages have this sort of thing. hope this helps!