In "The Gift of the Magi", Della stops at a sign that reads, "Mme. Sofronie. Hair Goods of All Kinds. One flight up." Is this an example of...

In order to answer this question, we should start with definitions. Situational irony is when incongruity appears between exception and what actually happens. A surprise ending is just like it sounds, when there is a surprising twist at the end of the story. 

If we start with these definitions, then Della's visit to Mme. Sofronie is an example of surprise ending, on account of what takes place.  Mme. Sofronie buys hair, and Della has the most beautiful hair. So, Della sells her hair to Mme. Sofronie for twenty dollars. She uses this money to buy Jim, her love, a gift. She winds up buying a chain for his prized watch.  

When the time comes to exchange gifts, Jim realizes that Della cut her hair.  He is surprised and taken aback, because he bought her combs for her hair.  Moreover, he sold his watch to buy the combs. In short, they cannot use their gifts.  This is the surprise ending.  She has little hair and no use for the combs.  He has no watch and no use for the chain.