Della is the character the story follows, and she's a woman with a mission: finding her beloved husband the perfect Christmas present. She's the character with whom we most identify. We suffer with her as she despairs of being able to buy the man she loves a good present. We also feel the weight of the sacrifice she makes.
But it's almost an accident that Della, and not Jim, is the protagonist, since their situations and their actions are essentially the same. It's just because the narrator chooses to follow Della that she's the main protagonist. If you focus on what the story means, Jim and Della are both protagonists, since they both face the same challenge and respond the same way. They learn together what it truly means to give.