It's Daddy's birthday, and what would a birthday be without a cake? Ma clearly knows this and sends Billie Jo to Mr. Hardly's town store to get the ingredients.
Mr. Hardly isn't exactly the model of customer service. He's kind of grouchy and frequently gives customers less change than he should. Billie Jo assumes his bad mood is because his attic filled up with dust and collapsed, destroying a large portion of his stock.
When she gets home, Billie Jo realizes that Mr. Hardly gave her four extra cents in change instead of cheating her like he usually does.
She returns to the store and puts the money on the counter, wondering if Mr. Hardly will reward her for doing the right thing. He doesn't, but Billie Jo is all right with this because Ma would never allow her to accept a gift from him.