Hey there! I believe your answer is Her wanting to buy a Christmas present for Jim. I know this because in the story, Jim sells his favorite watch to buy Della's favorite combs, and then she feels as though she owes him for that. So, by selling her hair, she can use the money to buy Jim a gift just as good as the one he got for her. I hope this helped!
The last sentence describes and conveys that the thoughts were racing through her mind at the SAME time she was calling her grandmother. This sentence helps the reader visualize the situation.