Even though Lisa lives under the same roof as Oscar, it doesn't mean that she is dependent on this man. All her needs, except for food, have been provided, but she still wants to buy things on her own. She doesn't forget to always send half of her salary to Kumala's account every month.
Lisa aspired to always live frugally. If the item was not important, he certainly would not buy it. She thought it'd be better to save the money just in case there was an urgent need.
Now, if Lisa buys William a toy from this shop, maybe she won't be able to take a shower with soap and eat lunch with just a side dish.
Lisa put the toy back, feeling sorry for the boy. If only she had as much money as Oscar, she wouldn't have thought twice and bought William all the toys the store sold.