The Question and Answer section for Oranges is a great resource to ask questions, find answers, and discuss the novel.
I think the line suggests that the boy knew the store clerk would understand that he did not have enough money for the chocolate that the girl picked. Indeed, the store clerk did understand and accepted the orange as partial payment for the candy....
I think the audience is simply the reader whoever they may be.
The woman understands that he doesn't have enough money for his purchase.... and she makes sure that he gets it. It's the importance of love.