on the line below explain the surprise ending and how o. henry makes it seem logical.

What line are you referring to?

First of all to Aslan, the line is on the homework/test the asker is completing.

The surprise ending is when the reader realizes the couple have exchanged gifts, but the cost of the gifts made the other gift worthless.

O'Henry shows that this couple is deeply in love and willing to do anything to show it, so they sacrifice their most important possessions to love each other and they are only unlucky that it turned out the way it did.