only a "large shed of corrugated iron," the prices are much higher than those in town. The shopkeeper is selling "hamburg" meat for 20 cents a pound when in the high street it costs only 15 cents a pound. Ma mentions how expensive it is to the shopkeeper, but he just laughs in her face. At one point, knowing she has no choice but to spend her money there, he tells her he wouldn't want to eat the food in the store himself.

Ma scolds the man for making fun of her, observing that he only does so because "doing' a dirty thing like this. Shames ya, don't it?" Cleverly, she breaks down his hard exterior by pointing out that he is just like them—a poor person doing what he has to do to survive. In her usual gentle manner, she persuades him to advance her some money for sugar.

He looked pleadingly at her. And then his face lost his fear. He took ten cents from his pocket and rang it up in the cash register.

This is a great question. A little context is important here. 

In this chapter, the Joads are in California and they have a job picking peaches. At first they are happy about the job, since they are in a desparate predicament. They get five cents per box. The family works all day long and they are able to make one dollar. With this money, Ma goes to buy dinner at the local store. 

When Ma arrives at the store, she realizes that the store has raised prices. So, not only do the Joads barely make any money for their labor, but also the food costs have risen. They are being hurt on two fronts. In the end, Ma does not have enough to buy food for one meal. And when she wants to buy sugar, the clerk would not give it to her on credit. In this dialogue the clerk who is also poor decides to put up the money himself. When this happens, Ma speaks these unforgettable lines:

"I’m learning one thing good," she said. "Learnin’ it all the time, ever’ day. If you’re in trouble or hurt or need-go to poor people. They’re the only ones that’ll help-the only ones."  

The Joads work at a peach orchard in chapter 26 of The Grapes of Wrath. They all pitch in with picking but only manage to earn a dollar. With a slip (voucher) worth a dollar, they can buy some much-needed food at the company store. Ma Joad goes there to get the groceries. The small, bald shopkeeper reminds her of a bird. She tells him that she has the dollar voucher, and he says she can choose anything.

As she indicates her selections, she is shocked at the high prices he quotes. He rationalizes the prices by saying it would cost her more in gas if she had to drive to the store in town. Ma sternly rejects this justification and critically eyes the hamburger meat before deciding to buy it. She continues arguing with the man over every purchase, and then changes her tone and asks him about his situation. He is not the owner, just another employee of the company. She realizes that his mocking attitude is his way of coping for always being challenged over prices that he does not set.

After she finishes shopping and settles up, she realizes that she had forgotten the sugar but has nothing left to spend. She asks the man for ten cents worth of sugar on credit, but he refuses, afraid that he will be fired for breaking the rules. Then he takes a dime out of his pocket and buys the sugar for her, saying she can pay him back with the next slip. Ma concludes,

If you’re in trouble or hurt or need, go to the poor people. They’re the only ones that’ll help.