I don't remember the name of this story, but I believe I do remember which Jim and Della we are talking about XD I'm sorry if I'm wrong.
Jim's gift to Della is an example of irony because Jim sold what was dearest to him to buy something worth remembering for Della, and Della sold what was dearest to her to buy something for Jim. They both gave up one of the most worldly things which meant dearly to them, for the one they loved.
Hope I got it right? Is it by O. Henry, the author who's known for his surprise endings?